Maybe someone has made schematics of the board that I'm able to retrace the signals.

The next step is to resolder the capacitors.

To my understanding:

AM I correct that to see the disk with "?" is a good sign or it has no meaning?

And the missing startup chime may have to do with badly soldered capacitors.

I have ordered "Apple Macintosh Level 1 schematics", but my fear is there is no Quadra schematics inside :-(