A bit late, but at the "too many Avatars argument" -
The difference between these characters and Mii is that they are pre-made characters meant for you to play as. Much like Link, who was also created as a player Avatar, they each have their own distinct appearance and abilities. The thing that makes Mii different is that it wears your freaking face and that it has no specific, set abilities. It's Nintendo's first truly customizable avatar.
Whether that helps or hurts its chances, I don't know. It might be playable, it might not be. I don't see Mii happening in Smash without some kind of customization, though, whether that be just costumes like in Mario & Sonic at the Olympic Winter Games or whole moveset changes. Perhaps there'll be a few preset movesets that are amalgamations of other characters. I dunno. The point is, excluding them because they're an Avatar and we already have one is a poor argument.