You missed an important point -- he's qualified to referee Quidditch matches. Snag a copy of "Quidditch through the Ages"; you'll see that it's *hard* to qualify as a referee.

You have to pass tests from the history of the Quidditch to know all the fouls; and you *must* be proficient on a broom to a certain level.

So Snape can fly. Probably not in Harry's league, but he's not going to fall off any minute either.