I think the picture could actually work as evidence for your theory as well. I mean, if River and the Doctor had a daughter and Amy and Rory were still around as companions you know they would become the kid's cool godparents or aunt and uncle. And maybe Amy got her picture taken when holding the baby because she was the honorary aunt? I feel like that's a plausible explanation for Moffat to use, if the story does break the way you're thinking.

Anyways, this was really awesome to read. As much as I generally shy away from fanfic in which people make River pregnant - because it's usually just so full of gender stereotypes that I find it nauseating - I think Moffat could pull it off and...man, that kid would be just epically awesome. No wonder she can break out of an astronaut suit and survive on her own!