Ooo, also, if Amy is in the spacesuit and shoots the Doctor, it means that River unknowingly shot at her mother (but missed) thinking that she was the one to kill the Doctor, and Amy unknowingly shot at her daughter (but missed) thinking the same thing.
I... did say that in the meta. ;) (Although it was janie_aire who first pointed it out.) And it's by far the most convincing reason for it being Amy!