Ooh, yes! The main thing that was bugging me in this theory was the question of how/why would the Doctor and River ever let something like that happen to their daughter? We have seen what the Doctor is like when his nearest and dearest are threatened; I find it incredibly hard to believe that daddy!Doctor would go, "Well, it's up to my past self! Tralala!" River is possibly even less likely to be okay with it. The idea that the River and Doctor who are the girls' parents no longer exist makes a lot more sense to me. (I also like that your theory deals a fatal blow to this whole "back to front" relationship thing, which is clearly nonsense - or at least, was certainly nonsense right up to the moment the Doctor got killed.)

Btw, when I linked to this from my journal, someone pointed out to me in a comment that this puts a whole new spin on who killed the Doctor. If it was his daughter and old!Eleven knew it, then his comforting her makes a lot more sense. And wow, is that really tragic and dark. I'm on the fence about whether DW would really go there.