Gbuffer size would reach unholy levels and consequently will munch your bandwidth. Nothing to do with what you do with your scene.
Deferred with MSAA is doable, but barely feasible even for minimal gbuffer.
Yeah, that its called post temporal syndrome. Welcome. People started to forget how a clear picture should look. On the other hand, the amount of junk you can throw at temporal aa and come out with a decent and usable picture is sky high.
No. Not always and not every fragment. Depends on implementation. Whole point of MSAA is to multi sample geometry, but shade at screen res rate, otherwise it would have been a simple super-sampling.