Also, there is the question what Frederick would do if his country gets broken up like that. Brandenburg would be a medium power like Bavaria and Saxony (it looks still bigger on the map, but it's thinly settled once you leave Berlin).
Might he even go as far as committing suicide? In that case, his nephew (OTL Frederick Wilhelm II) would become king. Note: That's the same guy who was nicknamed "fat playboy", who would be only 18 years old in 1762, and brought Prussia pretty much down during the eleven years he reigned IOTL. Noone will have to fear Brandenburg for the next generation if he becomes king.
Frederick is probably dead in the final battles that led to Br-Pr 's defeat. That would have been his intent had Empress Elizabeth not so fortuitously died. It will be his nephew who is left to pick up the pieces.