What they said is that we will see some returning NPCs from D1 & D2. Since the Adria concept is dated 2007, and D3 was already in development, we can say 100% sure that they have thought about adding her to the game, however noone knows if she will make it in the final release or how big her role will be. She could just aswell be an unimportant NPC added for nostalgia. Blizzard probably has multiple storyline ideas right now.
I haven't read anywere that Adria was spared from diablo, in cains diary he says everyone was killed. Meaning her too wouldn't it? It never said anywere that she packed up and left before Diablo came back.
Well, if she possessed the knowledge that even Cain did not (as Cain himself said that), then I guess she could know what's going to happen with Tristram! Maybe she has a gift of foresight! Maybe Leah is really her daughter, since she too, had visions in the Cinematic Trailer!