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?

On reflection - perhaps trying to combine all these elements in one night is a little ambitious. It would be difficult enough to pull this off with previous experience and knowing in advance the sort of issues that will arise in advance.

One night for the open mic, and for acts taking turns using the stage and another night where whoever turns up can join in together and not use the stage at all - is perhaps more realistic for a first attempt.