Yes to all three of your questions. However, like almost all of his other moves, Sonic's nair is easily outprioritized by most characters if they see it coming far enough ahead. It's definitely the most versatile aerial opener he has(it also has a sweet spot).
Sonic very rarely has anything better than a grab or dash attack that is truly safe to do without creating his own opening. Fortunately, his speed makes him extremely good at doing this; I'd have to say that in given at least a little space, Sonic is almost always at the offensive advantage in a grounded situation(since he is much slower in the air) because of how fast he can move and the fact that he is one of the only characters(if not the only) which has anything close to a Melee dash dance(aside from fox trotting/pivoting, which dont really carry the same functionality).
It's figuring out exactly how to use the opportunities you've created that's making problems for people with Sonic. Once people learn the spacing on his aerials and tilts better and such, and more combos are created(I've got one combo that is an ensured kill at 100% vs most characters, providing they don't DI right(whether or not it can be escaped with DI is thus far untested), I have a feeling Sonic will become one of the better characters.
Oh, and the movement, oh god, the movement. Not being able to do anything but attack and jump during the initial dash frames really kills a lot of approach methods most people will use coming from Melee. Once you stop dashing into people's attacks expecting to be able to shield or in general do anything useful at all(since getting into the air doesn't especially help sonic, he's not exactly Lucario or Marth), Sonic loses a lot of what usually gets him hit.
He's just too fast not to be good, honestly. People need to learn the rules for priority as it relates to spacing/zoning in the various matchups before he's going to get there, though, and that takes experience more than anything else(since hitboxes aren't something you can actually see(yet)).
