Okay, so: Frau Prinzessin, the princess, without any additional name is always Mina, Heinrich's wife. So, the passage you asked about in context goes:
"On the occasion of the Princess' birthday, the Queen invites the entire town to a ball and a beautiful party. I've just returned from it, and the music is still ringing in my ears. Earlier, I've dined at Princess Amalie's, who celebrated her sister-in-law's birthday. Recently, after she'd dismissed Herr v. Holtzendorff from her services, she has caused Frau v. Keith to provide her (A) with her son (i.e. Frau v. Keith's son) as a cavalier. Eight days later this young man was transferred to the cabinet, and thus she's again without a cavalier. Now she's entrusted herself to a quack whom she believes to be capable of performing miracles for her, because he recently cured a chamber woman of Princess Wilhelmina (Minor)'s from dropsy."
I.e. the son in Question is definitely Frau v. Keith's son. However, whether Frau v. Keith is also Peter's widow is another matter. And no, in the two pages on Peter apropos his dying, there's no mention of a son. Otoh it's mentioned his wife is a born Fräulein von Knyphausen, and the Knyphausens definitely have various Family members at court, so it could be her.
The Queen: Lehndorff usually means EC when speaking just of the queen; if he speaks of both her and SD in one Paragraph, he adds "the Young Queen" or "the reigning Queen". SD is more often the Queen Mother than she is the Queen, but just to make your life more difficult, sometimes she is the Queen, too. (For example when he compares the two to EC's disadvantage after SD's death.) Anyway, if there's just one queen he's referring to in a passage, without any additional Attribute, it's usually his boss.
Peter's place of death: the passage doesn't say, but instinctively I'd vote for "he died at home, not in war", because Lehndorff writes "around this time, Lietenent Co. Keith dies, too". Stirbt, dies, not "fällt" (falls) or "wird getötet ("is killed"), which is what's more commonly used for death in battle. Also worth considering: the description of Peter's career mentions Fritz put him in charge of the administration of Charlottenburg Palace and of the Tiergarten. Since this is at when the war is still going well for Prussia, there is no sign of evacuation, and the court is still in Berlin, which means Charlottenburg isn't going to administrate itself, and the Tiergarten is as popular a Destination for Berliners as ever.
Also worth noting: the immediate next entry for January mentions that all four Hohenzollern Brothers are in Berlin for a quick visit with their mother. This will be the last time SD sees all of her sons together and they see her alive. So if Peter did die in Berlin at the end of December 1756, Fritz might, in fact, have been there - not at his death bed, presumably Lehndorff would have mentioned that, but he could have seen him.
Then again, I could put a far too great importance on the verb "die". Still: for example, when Lehndorff mentions his older brother died in the aftermath of Hochkirch, not at the battle itself, he mentions the cause - "der rote Friesel" - which is a slang term for wound infection, and if he records battle deaths, he usually mentions the battle place in question. Especially if it's someone he has a good opinion of, which certainly is the case with Peter Keith.
Re: Peter Keith
"On the occasion of the Princess' birthday, the Queen invites the entire town to a ball and a beautiful party. I've just returned from it, and the music is still ringing in my ears. Earlier, I've dined at Princess Amalie's, who celebrated her sister-in-law's birthday. Recently, after she'd dismissed Herr v. Holtzendorff from her services, she has caused Frau v. Keith to provide her (A) with her son (i.e. Frau v. Keith's son) as a cavalier. Eight days later this young man was transferred to the cabinet, and thus she's again without a cavalier. Now she's entrusted herself to a quack whom she believes to be capable of performing miracles for her, because he recently cured a chamber woman of Princess Wilhelmina (Minor)'s from dropsy."
I.e. the son in Question is definitely Frau v. Keith's son. However, whether Frau v. Keith is also Peter's widow is another matter. And no, in the two pages on Peter apropos his dying, there's no mention of a son. Otoh it's mentioned his wife is a born Fräulein von Knyphausen, and the Knyphausens definitely have various Family members at court, so it could be her.
The Queen: Lehndorff usually means EC when speaking just of the queen; if he speaks of both her and SD in one Paragraph, he adds "the Young Queen" or "the reigning Queen". SD is more often the Queen Mother than she is the Queen, but just to make your life more difficult, sometimes she is the Queen, too. (For example when he compares the two to EC's disadvantage after SD's death.) Anyway, if there's just one queen he's referring to in a passage, without any additional Attribute, it's usually his boss.
Peter's place of death: the passage doesn't say, but instinctively I'd vote for "he died at home, not in war", because Lehndorff writes "around this time, Lietenent Co. Keith dies, too". Stirbt, dies, not "fällt" (falls) or "wird getötet ("is killed"), which is what's more commonly used for death in battle. Also worth considering: the description of Peter's career mentions Fritz put him in charge of the administration of Charlottenburg Palace and of the Tiergarten. Since this is at when the war is still going well for Prussia, there is no sign of evacuation, and the court is still in Berlin, which means Charlottenburg isn't going to administrate itself, and the Tiergarten is as popular a Destination for Berliners as ever.
Also worth noting: the immediate next entry for January mentions that all four Hohenzollern Brothers are in Berlin for a quick visit with their mother. This will be the last time SD sees all of her sons together and they see her alive. So if Peter did die in Berlin at the end of December 1756, Fritz might, in fact, have been there - not at his death bed, presumably Lehndorff would have mentioned that, but he could have seen him.
Then again, I could put a far too great importance on the verb "die". Still: for example, when Lehndorff mentions his older brother died in the aftermath of Hochkirch, not at the battle itself, he mentions the cause - "der rote Friesel" - which is a slang term for wound infection, and if he records battle deaths, he usually mentions the battle place in question. Especially if it's someone he has a good opinion of, which certainly is the case with Peter Keith.