1. Yes, and as far as I can tell it's very similar to な(あ).
2. a) Yes. It's like old person speak, I'd say. じゃ usually goes with that as well.
b) Flavor.
c) で has more of a feel of explanation to me. I'm curious to know if that's on the mark, though.
3. Yes, it's that again. I don't think it always means that, but I can't think of any counterexamples at the moment.