I can't understand why it's feeding back. Accordions are bloody loud, so you shouldn't have to drive the gain or have it blasting in Jim's monitor.
Mrs WoT used to use a Sennheiser E608, but these days she usually just uses a mic so she can control her volume. I can't recall a blast of feedback ever.
FWIW, you can roll shedloads of bottom off an accordion at a gig. It makes a heck of a difference to what's swimming around on the stage.