The only characters you can play are ones that you have hit level 70 with on some PvE realm somewhere.
Good grief, why? If you want a proper split, Warcraft Arena should mandate playing WoW at all, that makes no sense. In fact, there wouldn't even be a point in levels at all in a separate game. I fail to see the point of a split but if there were to be one, limiting players to their WoW mains is a good way to make a horrible idea even worse, no offense meant.
None take, don't worry about it ^^.
Eh, I suppose it'd be nice to force players to learn a class so to speak by going from 1-70. Gives new guys a reason to play through the game or something. Increases replay value. Call it whatever you like.
But in any case, you're absolutely right. All of that I said up there is a silly idea and would never work. I'm not a game designer nor will I ever be.