Mo, You will need a PA. It's not about the size of the room, it's about the amount of noise you get from people talking/shouting. You might get away without it at your local dance club but as soon as you have the excited atmosphere of a wedding, family or student party you'll be drowned out by the noise.

Insurance: your personal PLI won't do, but you can have the band join as an affiliated member of the EFDSS and get PLI that way, covering whoever is in the band + caller even if it's different people each time.

Sharing mics: don't. One concertina needs two mics, one on each side. If you want lots of small, cheap and perfectly serviceable mics, get some Behringer C2s. Barely over £40 a pair - your mixer will need to be one that supplies phantom power to the mics. I used a pair of C2s for my boxes and cello at the Frodsham concert with Jez Lowe - you were there, Mo : did they sound OK? Get them from Digital Village or Thomann, good places to shop around for the rest of your gear too.

Anahata