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.)

Good point.

A theory my sister and neice came up with is that, whoever she killed, the real reason she's in Stormcage is so Eyepatch Woman can't reach her.