Melody Pond can't be Spacesuit Girl, because River (who is Melody) was as baffled as anyone else about the spacesuit, why the little girl was calling for help, etc. (Or she's a good liar.)

I do think she might have killed Rory. Or possibly just witnessed his grizzly end, or outlived him, or something. She seemed surprised, and a bit sad, when he showed up at Stormcage.

I'm mostly still puzzled about why Amy and River both got stomach-sick on early exposure to the Silence. WHAT IF THEY WERE BOTH PREGNANT? Spacesuit Girl being River and the Doctor's kid or something would be just so... soap opera.