I'll tackle the second question first. Yes, godly domains/astral dominions are now "floating in the Astral Sea." However, the Astral Sea isn't really a sea per se; it's a great silvery void, much like it always was, but there is a noticeable "astral horizon" that travelers and vessels tend to orient toward, creating the impression that they're traveling along the surface of a sea. The clouds below that horizon tend to take on a darker, gunmetal-silver color, while the ones above are a little brighter. Stars glitter in the spaces between the clouds. Really, the Astral Sea is a medium in which the finite, unique astral dominions can exist.
The dominions themselves are not infinite planes any more. Some are pretty big (many hundreds of miles across), others might be as small as a single city or islet. The Nine Hells, for example, are continent-sized caverns within a ruddy, volcanic world adrift in the Astral.
As far as the effects of Cyric's imprisonment, the answer is, not much. Direct assistance from Cyric is harder to come by, but then again, Cyric-like most deities--usually works through proxies, angels, and mortal priests. Of course, it drives *Cyric* nuts, because he can't stand the idea of anyone telling him what he can and can't do. That means he's got a big score to settle.
I would like to second this. [request for info on cyric's imprisonment, astral domains]