Re: Fritz' illness at Küstrin - this reminded me of a detail I'd come across recently, in a letter from Hille to Grumbkow from January 1732:

The Crown Prince has been ill for days and looks miserable. I ascribe this to the cold [outside] and his ice-cold bedroom, in which he sleeps without a nightcap and with a very thin blanket.

I'd filed this away as one more data point for his propensity to keep his rooms very well heated in later years, but it's certainly also relevant for any h/c fic. :)