Interesting theory, but here's my understanding of things:
in River's personal timeline, in the one we see in LKH when she's just de-Mels'd/en-River'd, she hasn't killed the Doctor yet.
It's possible that she killed the Doctor later on, but remember that the astronaut suit was made for a child, and River has seemingly gotten over her programming. Then again, she was still in prison at one point for killing a "great man"
I think what makes me think robot is that the astronaut came out of the water, and disappeared back into it. If it was a time travelling robot full of mini people, it could do that.