The biography "August Wilhelm, Prinz von Preußen" from 2006 is by Eva Ziebura, who also more recently wrote a Heinrich biography my library doesn't have. But it does have "Die preußischen Brüder: Prinz Heinrich und Friedrich der Große" by Christian Graf von Krockow from 1996, which is an elegant double potrait/biography esay book not always told linearly and also pondering on Prussia per se. (The author, as he admits in the foreword, hailing from Prussian nobility himself and hence finding his own upbringing etc. influencing him on his take on the brothers.)
The AW biography: confirms Wilhelm or Guillaume was the name he actually used, with his younger brothers calling him Guille, does its source notes and tells its tragic story well, with the author unabashedly biased for her subject but trying to check it as in the question how far or little Fritz' behavior towards AW - when he basically wooed this younger brother with letters, even once wrote him a poem of praise - during the ten years between 1730 and 1740 was utterly calculating or not. It is, she admits, almost impossible not to read it with hindsight, but it's entirely possible Fritz was just as much motivated by affective needs - missing Wilhelmine and wanting to have an ally among the remaining in Berln siblings again - and survival technique (an ally who can report on their father's moods and within limits affect them); Zuibura does devote a powerful chapter on just how much Fritz was abused to demonstrate where he came from. Maybe he also was sincere in offering his older brother guidance re: education, not least because AW did need it; FW, who'd been delighted this second son prefered the toy soldiers to the books, had allowed his teacher to give him a great leash when it came to history, geography, writing etc, with the result that AW was on a level with the younger Heinrich (who was more like Fritz and Wilhelmine as a passionate learner). The poem of praise is also a gentle instruction on why books are cool and teen AW should read more of them on his own.
Young AW responded eagerly (and on that occasion btw tried to write a poem of his own to his older brother back). Unfortunately, all of this meant he really had no idea and post 1740 Fritz caught him entirely unawares. (BTW, the turnaround didn't happen immediately after ascension; for example, Fritz took AW with him and Algarotti when making that trip across the French border via Bayreuth, which is why we know - from AW's letters - one detail Fritz left out in his description to Voltaire about said trip, i.e. after his incognito was blown by a Prussian deserter, the lot of them ignominously spent the rest of the night under arrest (passport forgery!) until the local commandant cleared things up and send them back. (Young AW still thought this was a jolly adventure, though, and that his older brother was the coolest at this point.)
As Ziebura details, the reason why FW prefered this kid was a chicken/egg thing going beyond "that one thinks soldiers are cool" - because young Wilhelm was a loved child, he wasn't afraid of his father and very affectionate towards him, because he never had to hide what he thought, he was honest (whereas FW was convinced Fritz and Wilhelmine were lying to him all the time, which of course they did because they were terrified), because he was the golden child, he was generous and kind towards all his siblings and eager to help them whenever they asked, which in turn made him popular with the rest of the family as well. And Heinrich, who shared rooms with AW from the time he was four, really loved him above and beyond, same intensity as the Fritz/Wilhelmine relationship. You can see where all this is going.
Starting with the first Silesian War, Wilhelm has a wake up call in that him being the new crown prince does not mean Fritz is going to share foreign policy or any other plans with him - not then, not ever. And once Fritz is back, the bossing around starts in earnest. While AW does have the (het) sex life with mistresses Fritz scorns in princes anyway, he's by no means without the thing FW successfully drummed into all his children - the sense of duty and serving as the main purpose of a Prussian royal. He writes lengthy memoranda about improvements he'd make if he was King, for example. (Trivia: these, btw, include giving the Queen a larger budget than the Queen Mother - Fritz did the reverse - though AW was at best indifferent to his wife as well. But he did think the reigning Queen deserved to have the bigger household. Given an early anecdote is how SD threatened to have little AW whipped by rod if he didn't ask his father for a deserter's life - one of the long fellows who'd run away - , which he then successfully did, I'd venture Wilhelm might also have been less of a fan of his mother.) He also indulged in a one year long strategy game with Heinrich between the Silesian Wars and the 7 Years War. They correctly assumed yet another war would happen, though along with Fritz and most others, they did NOT see the Austria/France alliance coming, so their imaginary war was between a Prussia allied to France and an Austria allied to England. Also tellingly, Heinrich roleplayed Fritz while AW took the role of Field Marshal Gessler in this scenario. ("Le Marechal Gessler" was also much later a pseudonym Heinrich chose for a few Fritz critiques.) They did correctly predict - or Heinrich-playing-Fritz did - that he'd kick things off by invading Saxony.
Ziebura is definitely of the "Fritz was scapegoating AW for mistakes that were partially his" school when it came to the big disaster, and attaches in full an assessment of von Schmettau about Wilhelm's decision to withdraw. She also quotes at length from Amalie's report on AW's death lengthy and painful death. Amalie was present through most of it, and said report, in great detail, tearstained and with ink blots, is adressed to Fritz. It doesn't include a direct accusation, but Ziebura thinks the very fact she writes this detailed has a "this is your fault, now at least learn what it was like" subtext.
Heinrich, of course, did not need subtext. He also unequivaably did not blame a medical reason for AW's demise. He was fine with main text, writing to brother Ferdinand (the three youngest brothers were called "the divine trio" in Berlin, I learned) on June 20th: "Since eight days, I know of the fate of our unhappy brother, and since this time, I am suffering. I am trying to have patience, but I shall never forgeet my beloved broother, or the terrible reason for his death." And in the next letterer, he's even more explicit: "Our misfortune is terrible, but I admire your attitude. You are completely right in ascribing our beloved brother's deaath to grief. (..) This misfortune hails from the one who makes his entire country miserable annd drowns Europe in blood."
(Now tell us how you really feel about your older brother, Heinrich.)
Surprise factoid: the one person Fritz was able to write a non-infuriating condolence letter to was AW's neglected wife. Also, his promise in said letter to be a father to her kids does not inspire Ziebura to a sarcastic comment re: FW2 but lets her point out AW's daughter Wilhelmine did say about Fritz in her memoirs "He was for me a second father, and his affectionate behavior towards me never changed".
On to the "Fritz and Heinrich" double portrait/essay/book: that one contains among many other things excerpts from their correspondance, including from Heinrich's reply to Fritz' infamous condolence letter: In the terrible shock the death of my brother has caused, it would have been impossible fo rme to write to you about a subject which to me is incredibly painful if it hadn't pleased you to write the letter adressed to me. The feelings which move me right now are more powerful than reason. I keep seeing the image of the brother whom I loved so tenderly, his last hours, his death. Of all the sad changes and misfortunes life can offer and from which I have not been always spared, this is the most cruel and most terrible that could have struck.
Fritz writes back, trying again. Heinrich is not moved. I have sighed enough about the misunderstanding between you and my brother. Now you keep reawakening the memory and encrease my pain. Only the respect I owe you and my pain keep me silent, and I am not allowed to reply.
Just in case this isn't enough of an 18th century style "Fuck you, Fritz" (with the obelisk awaiting), von Krockow also quotes the letter about Heinrich's visit to Wilhelmine, which I had only seen paraphrased as "he saw she was dying, and so he didn't tell her about AW" in the Wilhelmine biography. In the actual text excerpt, well....
My Bayreuth sister has been close to death. She cannot write to you. I am afraid that she will not recover from this illness. She doesn't know about my brother's death yet, for one is justly concerned here that telling her about this news would destroy even the glimmer of hope for her survival.
In other words: Fritz, you know, the sibling YOU love? You've killed her along with the one I loved best as well. Think about that while you're in the field. Think long and hard.
(Heinrich: has learned the Fritzian lesson of how to deliver words that truly hurt and go to the heart with frightening efficiency.)
(And it occured to me that the nightmare Fritz told Henri de Catt about, of Wilhelmine (of all the people) accusing him not to love their father enough, might actually be a classic case of transference even more than I thought, with FW standing in for AW as well.)
Von Krockow is with Mildred that the Fritz/Heinrich relationship is basically an eerie RP of FW/Fritz, and also in this that if FW, long after his death, was the figure Fritz still wanted love and pride from as well along with wanting to be his opposite, Fritz was that very figure for Heinrich. He seems to have been absolutely indifferent towards their father, there's no remark, either good or bad, on the record. The one he hated passionately and knocked himself out to work for and kept writing at least once a week to while he was still alive and kept obsessing about after his death was Fritz.
Von Krockow is good about both brothers, and laudably isn't coy re: their sexual orientation; though it's not his main subject, he devotes a chapter to Fritz/Fredersdorff on the one hand and Heinrich/his various boyfriends on the other, from which I learned Heinrich in his old age finally managed to score one who wasn't yet another charismatic money waster but kind and devoted, a French emigré officer, Antoine Count La Roche-Aymon. Von Krockow quotes Fontane (from his Rheinsberg chapter - that travel book, I tell you!): "Beautiful, graceful, amiable, an old school chevalier in the best sense of the world, he soon moved into a position of trust, and then into a relationship of the heart with the prince, of the type the later had not been able to enjoy since Tauentzien." (A previous boyfriend.) "The Count appeared as a present from heaven to him, the evening of his life had arrived, but behold, the setting sun gave him once more a beam of warming light."
In his last will, revised a few months before his death, Heinrich had mentioned him as follows: "I express my urgent gratitude towards the Count La Roche-Aymon for the tender devotion he has shown towards me during all the time I was happy enough to have him near me."
Von Krockow's resumé that life, in the end, had been kind to Heinrich. (His chapter on the brothers' love lives does not, alas, include the hot page Marwitz episode, so I'm still in the dark about that guy's first name or just how he was related to Wilhelmine's treacherous lady-in-waiting.)
More trivia: Mildred, contemporaries did testify that Heinrich as an adult did pretend not to speak German, but they always say "pretend", i.e. no one believed this was actually true. As opposed to his brothers, he managed to visit Paris twice (once when Louis XVI had to stand sponsor for the big credit needed to pay Heinrich's boyfriend's debts), and the people he met were charmed (and found him less opinionated than Joseph and less of an irritaging chatterbox than Gustav, the two most recent royal visitors) and testified he spoke an elegant French - but with a distinct "Germanic" accent. (Not surprising, since all the Prussian royals were taught French by Huguenot emigré descendants who had themselves been born in various German principalitis.) (Fritz seems to have had something of an accent, too, at least if Voltaire is anything to go by, who mentions he had to point out that "opinion" isn't pronounced with a g at the end, and "tete" does not rhyme with "trompette".)
There is a lengthy description of Heinrich in his old age at Rheinsberg by a Count Henckel von Donnersmark (of whom the director of "The Lives of Others" is descended, btw) which is over three pages, so I can't quote it in a comment, but it does mention that when the hour got very late, Heinrich's "no, I don't speak German,not me" slipped, especially when the 7 Years War got discussed, and a favored phrase was "Das will ich Ihnen noch sagen" ("one more thing I want to tell you"). Like Fritz, he had the local theatre play only French plays, all the time, though less exclusively Voltaire focused. And he did look like a figure from an older world in the end, with his Ancien Regime fashion and wig (Heinrich lived into the 19th century, after all), an excentric gentlemen with impeccable manners till the very end.
More Book Reports: AW bio, Fritz and Heinrich double portrait/lengthy essay
The AW biography: confirms Wilhelm or Guillaume was the name he actually used, with his younger brothers calling him Guille, does its source notes and tells its tragic story well, with the author unabashedly biased for her subject but trying to check it as in the question how far or little Fritz' behavior towards AW - when he basically wooed this younger brother with letters, even once wrote him a poem of praise - during the ten years between 1730 and 1740 was utterly calculating or not. It is, she admits, almost impossible not to read it with hindsight, but it's entirely possible Fritz was just as much motivated by affective needs - missing Wilhelmine and wanting to have an ally among the remaining in Berln siblings again - and survival technique (an ally who can report on their father's moods and within limits affect them); Zuibura does devote a powerful chapter on just how much Fritz was abused to demonstrate where he came from. Maybe he also was sincere in offering his older brother guidance re: education, not least because AW did need it; FW, who'd been delighted this second son prefered the toy soldiers to the books, had allowed his teacher to give him a great leash when it came to history, geography, writing etc, with the result that AW was on a level with the younger Heinrich (who was more like Fritz and Wilhelmine as a passionate learner). The poem of praise is also a gentle instruction on why books are cool and teen AW should read more of them on his own.
Young AW responded eagerly (and on that occasion btw tried to write a poem of his own to his older brother back). Unfortunately, all of this meant he really had no idea and post 1740 Fritz caught him entirely unawares. (BTW, the turnaround didn't happen immediately after ascension; for example, Fritz took AW with him and Algarotti when making that trip across the French border via Bayreuth, which is why we know - from AW's letters - one detail Fritz left out in his description to Voltaire about said trip, i.e. after his incognito was blown by a Prussian deserter, the lot of them ignominously spent the rest of the night under arrest (passport forgery!) until the local commandant cleared things up and send them back. (Young AW still thought this was a jolly adventure, though, and that his older brother was the coolest at this point.)
As Ziebura details, the reason why FW prefered this kid was a chicken/egg thing going beyond "that one thinks soldiers are cool" - because young Wilhelm was a loved child, he wasn't afraid of his father and very affectionate towards him, because he never had to hide what he thought, he was honest (whereas FW was convinced Fritz and Wilhelmine were lying to him all the time, which of course they did because they were terrified), because he was the golden child, he was generous and kind towards all his siblings and eager to help them whenever they asked, which in turn made him popular with the rest of the family as well. And Heinrich, who shared rooms with AW from the time he was four, really loved him above and beyond, same intensity as the Fritz/Wilhelmine relationship. You can see where all this is going.
Starting with the first Silesian War, Wilhelm has a wake up call in that him being the new crown prince does not mean Fritz is going to share foreign policy or any other plans with him - not then, not ever. And once Fritz is back, the bossing around starts in earnest. While AW does have the (het) sex life with mistresses Fritz scorns in princes anyway, he's by no means without the thing FW successfully drummed into all his children - the sense of duty and serving as the main purpose of a Prussian royal. He writes lengthy memoranda about improvements he'd make if he was King, for example. (Trivia: these, btw, include giving the Queen a larger budget than the Queen Mother - Fritz did the reverse - though AW was at best indifferent to his wife as well. But he did think the reigning Queen deserved to have the bigger household. Given an early anecdote is how SD threatened to have little AW whipped by rod if he didn't ask his father for a deserter's life - one of the long fellows who'd run away - , which he then successfully did, I'd venture Wilhelm might also have been less of a fan of his mother.) He also indulged in a one year long strategy game with Heinrich between the Silesian Wars and the 7 Years War. They correctly assumed yet another war would happen, though along with Fritz and most others, they did NOT see the Austria/France alliance coming, so their imaginary war was between a Prussia allied to France and an Austria allied to England. Also tellingly, Heinrich roleplayed Fritz while AW took the role of Field Marshal Gessler in this scenario. ("Le Marechal Gessler" was also much later a pseudonym Heinrich chose for a few Fritz critiques.) They did correctly predict - or Heinrich-playing-Fritz did - that he'd kick things off by invading Saxony.
Ziebura is definitely of the "Fritz was scapegoating AW for mistakes that were partially his" school when it came to the big disaster, and attaches in full an assessment of von Schmettau about Wilhelm's decision to withdraw. She also quotes at length from Amalie's report on AW's death lengthy and painful death. Amalie was present through most of it, and said report, in great detail, tearstained and with ink blots, is adressed to Fritz. It doesn't include a direct accusation, but Ziebura thinks the very fact she writes this detailed has a "this is your fault, now at least learn what it was like" subtext.
Heinrich, of course, did not need subtext. He also unequivaably did not blame a medical reason for AW's demise. He was fine with main text, writing to brother Ferdinand (the three youngest brothers were called "the divine trio" in Berlin, I learned) on June 20th: "Since eight days, I know of the fate of our unhappy brother, and since this time, I am suffering. I am trying to have patience, but I shall never forgeet my beloved broother, or the terrible reason for his death." And in the next letterer, he's even more explicit: "Our misfortune is terrible, but I admire your attitude. You are completely right in ascribing our beloved brother's deaath to grief. (..) This misfortune hails from the one who makes his entire country miserable annd drowns Europe in blood."
(Now tell us how you really feel about your older brother, Heinrich.)
Surprise factoid: the one person Fritz was able to write a non-infuriating condolence letter to was AW's neglected wife. Also, his promise in said letter to be a father to her kids does not inspire Ziebura to a sarcastic comment re: FW2 but lets her point out AW's daughter Wilhelmine did say about Fritz in her memoirs "He was for me a second father, and his affectionate behavior towards me never changed".
On to the "Fritz and Heinrich" double portrait/essay/book: that one contains among many other things excerpts from their correspondance, including from Heinrich's reply to Fritz' infamous condolence letter: In the terrible shock the death of my brother has caused, it would have been impossible fo rme to write to you about a subject which to me is incredibly painful if it hadn't pleased you to write the letter adressed to me. The feelings which move me right now are more powerful than reason. I keep seeing the image of the brother whom I loved so tenderly, his last hours, his death. Of all the sad changes and misfortunes life can offer and from which I have not been always spared, this is the most cruel and most terrible that could have struck.
Fritz writes back, trying again. Heinrich is not moved. I have sighed enough about the misunderstanding between you and my brother. Now you keep reawakening the memory and encrease my pain. Only the respect I owe you and my pain keep me silent, and I am not allowed to reply.
Just in case this isn't enough of an 18th century style "Fuck you, Fritz" (with the obelisk awaiting), von Krockow also quotes the letter about Heinrich's visit to Wilhelmine, which I had only seen paraphrased as "he saw she was dying, and so he didn't tell her about AW" in the Wilhelmine biography. In the actual text excerpt, well....
My Bayreuth sister has been close to death. She cannot write to you. I am afraid that she will not recover from this illness. She doesn't know about my brother's death yet, for one is justly concerned here that telling her about this news would destroy even the glimmer of hope for her survival.
In other words: Fritz, you know, the sibling YOU love? You've killed her along with the one I loved best as well. Think about that while you're in the field. Think long and hard.
(Heinrich: has learned the Fritzian lesson of how to deliver words that truly hurt and go to the heart with frightening efficiency.)
(And it occured to me that the nightmare Fritz told Henri de Catt about, of Wilhelmine (of all the people) accusing him not to love their father enough, might actually be a classic case of transference even more than I thought, with FW standing in for AW as well.)
Von Krockow is with Mildred that the Fritz/Heinrich relationship is basically an eerie RP of FW/Fritz, and also in this that if FW, long after his death, was the figure Fritz still wanted love and pride from as well along with wanting to be his opposite, Fritz was that very figure for Heinrich. He seems to have been absolutely indifferent towards their father, there's no remark, either good or bad, on the record. The one he hated passionately and knocked himself out to work for and kept writing at least once a week to while he was still alive and kept obsessing about after his death was Fritz.
Von Krockow is good about both brothers, and laudably isn't coy re: their sexual orientation; though it's not his main subject, he devotes a chapter to Fritz/Fredersdorff on the one hand and Heinrich/his various boyfriends on the other, from which I learned Heinrich in his old age finally managed to score one who wasn't yet another charismatic money waster but kind and devoted, a French emigré officer, Antoine Count La Roche-Aymon. Von Krockow quotes Fontane (from his Rheinsberg chapter - that travel book, I tell you!): "Beautiful, graceful, amiable, an old school chevalier in the best sense of the world, he soon moved into a position of trust, and then into a relationship of the heart with the prince, of the type the later had not been able to enjoy since Tauentzien." (A previous boyfriend.) "The Count appeared as a present from heaven to him, the evening of his life had arrived, but behold, the setting sun gave him once more a beam of warming light."
In his last will, revised a few months before his death, Heinrich had mentioned him as follows: "I express my urgent gratitude towards the Count La Roche-Aymon for the tender devotion he has shown towards me during all the time I was happy enough to have him near me."
Von Krockow's resumé that life, in the end, had been kind to Heinrich. (His chapter on the brothers' love lives does not, alas, include the hot page Marwitz episode, so I'm still in the dark about that guy's first name or just how he was related to Wilhelmine's treacherous lady-in-waiting.)
More trivia: Mildred, contemporaries did testify that Heinrich as an adult did pretend not to speak German, but they always say "pretend", i.e. no one believed this was actually true. As opposed to his brothers, he managed to visit Paris twice (once when Louis XVI had to stand sponsor for the big credit needed to pay Heinrich's boyfriend's debts), and the people he met were charmed (and found him less opinionated than Joseph and less of an irritaging chatterbox than Gustav, the two most recent royal visitors) and testified he spoke an elegant French - but with a distinct "Germanic" accent. (Not surprising, since all the Prussian royals were taught French by Huguenot emigré descendants who had themselves been born in various German principalitis.) (Fritz seems to have had something of an accent, too, at least if Voltaire is anything to go by, who mentions he had to point out that "opinion" isn't pronounced with a g at the end, and "tete" does not rhyme with "trompette".)
There is a lengthy description of Heinrich in his old age at Rheinsberg by a Count Henckel von Donnersmark (of whom the director of "The Lives of Others" is descended, btw) which is over three pages, so I can't quote it in a comment, but it does mention that when the hour got very late, Heinrich's "no, I don't speak German,not me" slipped, especially when the 7 Years War got discussed, and a favored phrase was "Das will ich Ihnen noch sagen" ("one more thing I want to tell you"). Like Fritz, he had the local theatre play only French plays, all the time, though less exclusively Voltaire focused. And he did look like a figure from an older world in the end, with his Ancien Regime fashion and wig (Heinrich lived into the 19th century, after all), an excentric gentlemen with impeccable manners till the very end.