It lives! The screens I ordered are taking their sweet time getting here, but by all accounts it looks as if I have a functioning mainboard! I ended up doing a new PCB and just trying the FFC connectors again until I got the hang of it, and I think the mainboard and faceplate are all correctly soldered now.
edit: it plays music successfully, so everything seems good now save for the missing screen!
edit: it plays music successfully, so everything seems good now save for the missing screen!