Re: the kneeling - I have my info also from Stollberg-Rilinger's biography, who said that MT's Dad had introduced the three genuflections from his time as King of (Northern) Spain.
Cahn: MT's Dad had been the brother of the previous Emperor. This was just when the last Spanish Habsburg King, Charles of the minimal number of ancestors, died, and the Bourbons went after the Spanish Throne while the Austrian Habsburgs tried to hold on to it. Hence MT's dad for a while being King of Spain while his brother was HRE. Eventually, Louis XIV succeeded in putting a Bourbon on the Spanish Throne for good and MT's Dad went home, but not without Spanish Court Etiquette in his luggage, and since he became the next HRE, this meant he could inflict said protocol on all the German princes, to much resentment. MT upon her own ascension reduced it to one kneefall again, which I think it remained for a first encounter between FS, her and someone who hadn't been introduced before to them. In an encounter with her on her own, it was additionally helpful that she was a woman, because kissing a princess' hand was the polite thing to do in any case, even if she hadn't outranked everyone else.
re: what Fritz got out of the Wittelsbach Emperor - well, remember, MT didn't recognize the guy as Emperor. She pointed out that he hadn't been voted for by all the princes elector. Which was true but a bit of legal haggling (as was Fritz wanting his cake and eat it, i.e. claiming more independence from the HRE and saying he, he was warring againt MT only in the service of his Emperor, as a loyal Prince of the HRE), given there had been any number of cases in the middle ages where the Emperor hadn't been elected by all the princes. In any case, cousin Karl Albrecht was just "the Ursurper" to her, she never accepted him as HRE, so I suppose it's possible she also ignored all legal changes he made re: Prussia. Not sure about this, though. It's equally likely her pragmatic self accepted them after his son Max, Maria Antonia's brother, agreed to her terms (Bavaria back against dropping of all Wittelsbach claims to the HRE and vote for FS as Emperor in his capacity as the Prince Elector of Bavaria).
In any event, since Prussia voting for FS as Emperor after the fact had been part of the peace conditions post second Silesian War, and Fritz voted for Joseph post 7 Years War, again in his capacity as Elector of Brandenburg, as part of the peace conditions there, you have a mutual recognition by deed that a) Brandenburg-Prussia isn't an outlaw anymore, and b) it still recognizes the HRE as the supreme organization, headed by the Habsburgs for the foreseeable future, and Joseph as the next Emperor.
Re: Fritz cancelling the prayers for the Emperor in the 1750s: ironically enough, as we know from Lehndorff's diaries, he apparantly didn't think to cancel prayers and mourning for the Emperor's relations. I still can't get over the fact the entire Prussian Court wears full mourning when Isabella dies in 1763. (And I think Lehndorff also mentions prayers said for her.) Directly after the 7 Years War. For Isabella, who isn't related to the Hohenzollern at all, and whose only claim to said mourning is that she's the wife of the future Emperor. (The Court also wears mourning for Franzl in 1765, but that's a bit less strange - two years later, and he was the Emperor.)
Hypophetical summit: firstly, in the real summit, Volz has the complete version of Joseph's letter to Mom about meeting Fritz at Neisse in "Gespräche", as opposed to Jessen's shorter version which I had translated for you, and in it, Joseph does mention he encountered Fritz on the stairs (as depicted in Menzel's painting) and embraced him, Heinrich and future FW2 (whom Fritz had brought along); they then went upstairs and he and Fritz had a one and one chat (starting their 16 hours per day talks) alone in a room. This stairs thing, and Fritz coming down, solves of course the problem of protocol, as Fritz can't very well kneel when his Emperor is already embracing him, plus Joseph as the younger man can play it as politenesss. This is not an option for an MT-Fritz encounter where Joseph is still an Archduke. Now of course in theory FS - who is the actual Emperor, renember, MT officially is only Empress as the consort of the Emperor, hence "Queen-Empress" if the Prussians want to be polite and "Queen of Hungary" if they don't - could do what Joseph did and go for a monarchical embrace before Fritz can either refuse to kneel down or kneel. Especially if he plays on the fact they already met. But that doesn't solve the MT and Fritz question, and Franzl being loyal, I don't think he'd do anything to sabotage whatever line she chooses to take.
I don't think she'd stick to protocol over common sense, if she agreed to peace negotiations to begin with, so no, she wouldn't insist on the kneeling, but hand kissing is still on. It does have the advantage of being an expected male noble to female noble gesture on the one hand, but on the other, from a prince of the HRE to the de facto head of the HRE, is a submissive gesture. (During the 30 years war, several Protestant princes made a great deal of refusing to kiss the Catholic Emperor's hand.)
Re: Prussia and the Holy Roman Empire
Cahn: MT's Dad had been the brother of the previous Emperor. This was just when the last Spanish Habsburg King, Charles of the minimal number of ancestors, died, and the Bourbons went after the Spanish Throne while the Austrian Habsburgs tried to hold on to it. Hence MT's dad for a while being King of Spain while his brother was HRE. Eventually, Louis XIV succeeded in putting a Bourbon on the Spanish Throne for good and MT's Dad went home, but not without Spanish Court Etiquette in his luggage, and since he became the next HRE, this meant he could inflict said protocol on all the German princes, to much resentment. MT upon her own ascension reduced it to one kneefall again, which I think it remained for a first encounter between FS, her and someone who hadn't been introduced before to them. In an encounter with her on her own, it was additionally helpful that she was a woman, because kissing a princess' hand was the polite thing to do in any case, even if she hadn't outranked everyone else.
re: what Fritz got out of the Wittelsbach Emperor - well, remember, MT didn't recognize the guy as Emperor. She pointed out that he hadn't been voted for by all the princes elector. Which was true but a bit of legal haggling (as was Fritz wanting his cake and eat it, i.e. claiming more independence from the HRE and saying he, he was warring againt MT only in the service of his Emperor, as a loyal Prince of the HRE), given there had been any number of cases in the middle ages where the Emperor hadn't been elected by all the princes. In any case, cousin Karl Albrecht was just "the Ursurper" to her, she never accepted him as HRE, so I suppose it's possible she also ignored all legal changes he made re: Prussia. Not sure about this, though. It's equally likely her pragmatic self accepted them after his son Max, Maria Antonia's brother, agreed to her terms (Bavaria back against dropping of all Wittelsbach claims to the HRE and vote for FS as Emperor in his capacity as the Prince Elector of Bavaria).
In any event, since Prussia voting for FS as Emperor after the fact had been part of the peace conditions post second Silesian War, and Fritz voted for Joseph post 7 Years War, again in his capacity as Elector of Brandenburg, as part of the peace conditions there, you have a mutual recognition by deed that a) Brandenburg-Prussia isn't an outlaw anymore, and b) it still recognizes the HRE as the supreme organization, headed by the Habsburgs for the foreseeable future, and Joseph as the next Emperor.
Re: Fritz cancelling the prayers for the Emperor in the 1750s: ironically enough, as we know from Lehndorff's diaries, he apparantly didn't think to cancel prayers and mourning for the Emperor's relations. I still can't get over the fact the entire Prussian Court wears full mourning when Isabella dies in 1763. (And I think Lehndorff also mentions prayers said for her.) Directly after the 7 Years War. For Isabella, who isn't related to the Hohenzollern at all, and whose only claim to said mourning is that she's the wife of the future Emperor. (The Court also wears mourning for Franzl in 1765, but that's a bit less strange - two years later, and he was the Emperor.)
Hypophetical summit: firstly, in the real summit, Volz has the complete version of Joseph's letter to Mom about meeting Fritz at Neisse in "Gespräche", as opposed to Jessen's shorter version which I had translated for you, and in it, Joseph does mention he encountered Fritz on the stairs (as depicted in Menzel's painting) and embraced him, Heinrich and future FW2 (whom Fritz had brought along); they then went upstairs and he and Fritz had a one and one chat (starting their 16 hours per day talks) alone in a room. This stairs thing, and Fritz coming down, solves of course the problem of protocol, as Fritz can't very well kneel when his Emperor is already embracing him, plus Joseph as the younger man can play it as politenesss. This is not an option for an MT-Fritz encounter where Joseph is still an Archduke. Now of course in theory FS - who is the actual Emperor, renember, MT officially is only Empress as the consort of the Emperor, hence "Queen-Empress" if the Prussians want to be polite and "Queen of Hungary" if they don't - could do what Joseph did and go for a monarchical embrace before Fritz can either refuse to kneel down or kneel. Especially if he plays on the fact they already met. But that doesn't solve the MT and Fritz question, and Franzl being loyal, I don't think he'd do anything to sabotage whatever line she chooses to take.
I don't think she'd stick to protocol over common sense, if she agreed to peace negotiations to begin with, so no, she wouldn't insist on the kneeling, but hand kissing is still on. It does have the advantage of being an expected male noble to female noble gesture on the one hand, but on the other, from a prince of the HRE to the de facto head of the HRE, is a submissive gesture. (During the 30 years war, several Protestant princes made a great deal of refusing to kiss the Catholic Emperor's hand.)