This is a professionally installed job. The sound quality is great but the issues you mentioned are the big ones.
the reeds will vibrate when they pickup sound from monitors as well. Also, the cavity will tend to resonate and cause feedback when excited by sound from outside the instrument.
Would a lavalier mic or a mini PZM mic, if such a thing exists, be a good choice? i know nothing about their feedback sensitivity since i've only used them in video and studio situations. what about something like this?
Would it be better or worse to send the output to more sources? We could DI it to the PA and run parallel into a bass amp for stage volume...
So far I'm leaning towards the EQ or feedback squasher idea. afaik, the output on the accordion is unbalanced @ -10 db, with a volume knob. It worked well at a club when we DI'd it to the PA for FoH, but the monitors were causing the same feedback problem.