I know I harp on about this, but I think there's something worth considering in the Singdanceandplay workshops model:
http://www.singdanceandplay.net/free-traditional-music-classes-at-the-gun-and-spitroast-horsmonden/I go for all instruments, partly because I can being a multi-instrumental sort of person and partly because we're in a rural area where recruiting sufficient melodeon players would seem a bit unlikely. Also, going all-instruments makes sessions a bit more varied. If some of that stuff doesn't apply, the approach might well work for a melodeons-only regime.
During the season, which runs from autumn to spring, we run two workshops a month. I think regular playing with the tutor group is important. The workshops are free, but we do ask for contributions of £1-2 to cover expenses for events, such as hall hire, callers and advertising.
When workshoppers reach a point where they're happy to do so, they can join our once-monthly tunes session, where folks in turn nominate the tune they's like to play - I think there's a nice reward in practising something and then going out to play it in a session.
And then, twice a year, there's a dance at which the workshoppers are the band.
The workshoppers love it, some come from a way away and say they can't find anything like it nearer their homes, and a good handfull say it has transformed their playing. It's also been rewarding, fun and sociable for the workshop leader. I've always had many musical friends spread across the country, but never before have I been on friendly terms with so many people in my own community.
The teaching skills thing deserves and email all of its own, and I started from a position of not being entirely confident about the teaching, although I had quite a few experiences of festival workshops on concertina and fiddle to draw on. I can't say I found a great deal to direct me on the best approaches and EFDSS weren't able to help much either, but I'd say some of the usual management skills things apply. Praise where you can; show in whichever way you can that seems to work best; don't just show things but get folks to do them, or the they won't always sink in; for inspiration point out good examples from outside the classroom; and so on...
Gavin