Speaking of Lehndorff, I see elsewhere you‘ve found the info you wanted already, but just to make sure, I reread the January 1757 entries from Vol2, and they say Fritz and Heinrich were supposed to arrive on January 5th, but already arrive on January 4th at 11 pm at night and go and say hello to SD and the princesses anyway. (Lehndorff notes both Fritz and Heinrich kiss everyone on the cheek, which was of interest to me because I did wonder whether female family members were greeted by handkiss or cheek kissing+.) AW and Ferdinand don‘t arrive until January 11th, which is when they all have dinner at SD‘s, but then Fritz and Heinrich leave again while AW and Ferdinand stay. So in historical reality, Fritz alas was definitely not in Berlin when Peter died, but it really was just a few days, and I think you‘re more than entitled to claim some fictional license.

+ „Als er erfährt. daß die Königin-Mutter noch bei Tisch ist, geht er alsbald dorthin und erregt eine außerordentliche Freude. Er küßt alle Prinzessinnen und wird wieder geküßt. Der Prinz Heinrich macht es genau wie der König, und so sieht man nur Wange an Wange. und alles spricht zu gleicher Zeit, ohne daß man sich versteht; kurz, es herrscht große Freude.“