1) do you think we can successfully run an eclectic session with some folk, bluegrass, celtic and even singer-songwriter, jazz and oldtime folks thrown in for good measure?

Yes it is possible. But perhaps starting what you don't intend to be conventional performance - with a conventional performance (with your band) is not the best idea. As this will only confuse all concerned (especially other bands looking for oppotunities for paid gigs) and possibly make returning to the non-performance situation very difficult - if not impossible.

From the start - all parties and especially the venue - should have a clear idea what the concept is intended to be and just as importantly what it is NOT intended to be. If this means a slow start - then so be it.

But once the concept is understood - it should grow and hopefully keep everyone reasonably happy as long as the disadvantages of trying this eclectic approach are recognised from the start. For example It is difficult to marry an open mic (and a stage) with an all-join-in session, but not impossible.