Me too! The things we don't know for sure (or at least I don't, because I've seen contradictory sources and also sources stating we don't know) are: 1) did Fritz actually meet Fredersdorf and request him by name, or did Schwerin surprise him with Fredersdorf when he found out he wanted someone to play flute for him? 2) How enthusiastic or even involved in the decision was Fredersdorf? Obviously he came to care about Fritz eventually, but was it a case of "Yes, please, I'd love to go play for the Crown Prince I've never met, awesome job prospects there," "Yes, please, I'd love to go play for the Crown Prince who seems awesome in person," or "Um, I guess I live in Küstrin now, better make the best of it and try not to get my head chopped off"?

clearly I need to write more fic about Fredersdorf in order to retain this stuff

No shortage of co-authoring opportunities, just saying!