Personally, I really like the idea of Leah being Diablo in some way, and from the clips removed from the anniversary vid, it sure seems like something along these lines proposed is going to happen. How it will all transpire is the mystery now.
As far as it being bad story telling, I do see what you mean. And of any game fans out there, Diablo fans seem very protective over their lore and preserving the importance of past games. Even with the release of D3 coming there are still topics creating about D1, and even more about D2.
As a writer, I enjoy these kinds of twists and turns, it gives depth. But, when these kinds of things are "accidentally" revealed too early, like the leaked clip, and conclusions like these can be made, it sort of ruins it.
Imagine your surprise to find it out during the game, how truly shocking it could really be. But that can't happen now with all this speculation. So I can see why it would kinda suck to end up happening in the game.
However, that small little clip is not enough to really tell us much at all, and there are many things that could happen. Leah is definitely involved in some way, and from what I can gather there is no other explanation for that model of a female looking Diablo. Except for that one character that was sc!@*@d but then put back in as someone else, but idk much about who that was so its possible that doesn't fit at all.
Personally I'm really excited about the story of this game, it seems like it will be the most involved story, there's so much lore interspersed through the game, that it seems like we could find out quite a bit by gathering it all and piecing it together.