1. Due to the Space Colony ARK levels in SA2, Sonic and friends can breathe in space? 2. Were the SA3 screenshots that you showed in April an April Fool's Joke? 3. In Shadow's level, Sky Chase, on hard mode, was there a secret upgrade that you can get?
1. I'm assuming there's some sort of air-containment field around the ARK. In Rouge's Mad Space, you can see a barrier ring around the lower areas of the level which might be a generator of sorts. Plus, all the walkways (Sonic and Shadow's levels) and equipment (the trucks in Knuckles' Meteor Herd) seem to indicate that those areas were meant to be accessible for maintenance. Or, of course, you could just say "it's a game."
2. Yes, they were fake.
3. Shadow has no level called Sky Chase, so I'm guessing you mean either Sky Rail or Final Chase. Either way, both have an upgrade, but it's there no matter which mission you're on.

~ Crazy C Lea