Even if River was the girl in the alley,there is little chance she was the one who in the space suit who killed the Doctor,beacause she obviously can't interfere with herself and she tried to injure the space-suit person.
River Song is no doubt the little girl in the spacesuit, but the person she killed wasn't the Doctor because it was someone who was close to the Doctor.
It wasn't River in the space suit at the time obviously.
Additionally, there's evidence that the spacesuit can operate on its own and the young girl (presumably River, as there are no clues to it being anyone else) was a prisoner. Also, the Doctor told whoever was in the suit that he knew who it was. He made no attempt to resist, so it can be taken that he was OK with it.
I suspect the suit or whoever was in it had to fulfill some pre-programmed mission to kill the Doctor and once it was done he/she was able to break out of it. This whole thing has the stink of one of the Doctor's long term plans. The Seventh Doctor was especially fond of these.
Yes. If you doubt the Doctor's capacity to be "well devious" (as Ace put it), take a look at what Seven got up to.