We can safely assume that because Midgar isn't just a giant crater that Meteor never made contact.

As for Sephiroth being the guy in the chair... I would have argued about who it was a while ago, but with the information given, i.e. the wheel-chair guy being refered to as Mr. President, I think we can safely agree it's Rufus.

There is a chance that Sephiroth is alive, but when you slowly degenerate into nothing, I think you're dead.

The way I see it is that there's only one way to plausibly bring Sephiroth back to life: a perfect clone was made. Perfect in the sense of a complete genetic replication. Yazoo, Loz and Kadaj obviously are successful clones, but hardly perfect ones.