I'm no melodeonist so can't help with instrument specifics, but I've been using Microvox for years on winds and now also (English) concertina.

Putting the Microvox line out to a DI box, then XLR from the DI to the sound desk, works wonders for any volume issues. A passive DI box is fine, so there's not huge expense or batteries / mains to be concerned with.

I've run that way into all shapes, manners and sizes of sound rigs down the years with no complaints from the assorted denizens of the sound desks, who as a species aren't usually noted for being shy about expressing an opinion about the quality and level of signal they're getting :)

Feedback I can't comment on, although of course any mic system can be made to howlround if you get your mic placement, monitoring and FOH set up wrong / right ...