On a)
Different types of Communities are coming. I think there are 3 planned. The open Community which is what we have now, a community with whitelisted authors but where anyone can comment and a community where only the whitelisted authors can post or comment. Not sure when they're going to come out, but they are on the roadmap.

In the meantime, the Community Admin can mute posts and hide them from visibility so the sort of attack you mentioned could be stopped.

b) This one is an issue and I hope it gets resolved. I think the main thing is that the Communities are ranked by the payout of the posts in their Community, so what we would need is an option to "Resteem" into other Communities, but that could also pose it's own set of problems, mostly with spam control.

c) Yea, hope there is a better solution to this as well soon.