It's funny because I was just talking about this stuff with someone the other day. Yes she is in the younger body. Capcom probably did this because we are moving into a younger generation of heroes and they probably want her to stay around for a while. Revelations 2 is really kind of a late origin story to physically introduce her into the lore.
Due to plot armor and story telling. Capcom will not let her murder the whole Barry residence and burn it to the ground as
@Turo602 has suggested. It just won't happen. We may see Barry die, Polly.... doubtful. Moira will likely continue on to be a new fresh hero for the franchise along with Jake and Sherry. The idea of Chris's death being in relation to Alex would be an interesting scenario.
I will stand by my belief that Alex is the most evil person in the series. The documents in the game go into some very... disturbing details. Example. Manipulation, having peoples arms cut and burnt off, stitching people together, purposely testing peoples threshold of mental and physical torture. Her puzzles are the sickest in the series. She's also the only villain in the series to specifically target someone. The things she's done to hurt Natalia goes beyond forgiveness. Barry said it best, there is no salvation for her. I could go on.
As for how she will escape. There are plenty of scenarios. Running away or killing them is just one. If she wanted to create a suspicion. I have suggested that she could have Ada "kidnap" her. It would be all so easy. Alex let's Natalia's consciousness take over when it happens.... and there would be no evidence left behind to show it was a set up. That's just one theory of course.
As for
@Dr.Slaughter question. Of what she will be up to next. We have two hints in the game. One was the mention of her creating a "New World Order". Not like Albert's stupid idea, but more inline of what Spencer wanted. To rule over mankind as a god. While I doubt this will turn out to be the case but.... The "Tall Oaks" events is a direct lead straight to The Family. A group who could enact such a thing in a short period of time. The second thing. Is to engulf the world in fear. This would of course be likely through the use of Bio-Terrorism unless Capcom wants to do something scary with technology. Alex embraces the best of both as is stated in Revelations 2.
Another possibility is that she is the one who revives Umbrella, for what ever reason. As we all know Umbrella Corps takes place in the "current" cannon. Which Capcom defines by a year or two from the real worlds release date.
Alex Wesker was NOT a wasted character. Capcom created a character who is the most flexible character in the series to date. There is so much potential and Capcom can do basically anything with her. She could always change vessels again if Capcom wants to go a different route in the future. She's easily one of the most flexible characters in gaming history when you really think about it.