I think your daughter should decide. She's 16, she should have enough power at this point to make her own decisions. As long as it's safe, why not let her decide where to go?
Forget about whether it will salve the woulds of your ex for a moment. What would it do for your daughter, for her to have at least a little control in this crazy situation? It might be really healthy for her. The only balance point with be your boys - will they feel even more loss if your daughter follows your wife out the door?
Now getting back to the ex- if she wants a daughter around to help her with depression and guilt - what is so wrong about that? I don't think her interests top the list - I think it's the kids. But if you decide that for the good of the kids your daughter should go, and then your ex gets some benefit out of it, what's wrong with that? That's a mutually supportive relationship. That sounds delicious to me.
