About the potential new requirement to disable payouts for "council" type communities, would they affect the payouts in SMTs, if a community decides to have one which would practically be closed to members only? Or that would apply only to the STEEM reward pool?

Good question. Ideally this would require disabling STEEM payouts only. I guess it depends on SMT implementation. But currently this is achieved by setting max_accepted_payout to 0.000 SBD, and I don't think its likely that this will apply to SMTs, because it would be complicated to get the market value for each, especially when many will be illiquid. So I think we're good.

will there be room in the future development of communities as you envision them for what I've seen requested a few times, the possibility to switch guest posting on and off within a community?

We'll have new community types which restrict who can post, but I don't think we can have it be editable at will. At least not initially. There are edge cases to consider for the backend, and the goal is to keep the protocol dead simple (and efficient) for now.