I think the story was most likely pretty well set when the police first scoped out the scene. The rumors changed as bits and pieces of the official story came to light.
As I understand it, what was obvious after rudimentary forensics analysis was that he was shot 4 times; she was shot once, and had powder burns on her hand. Granted the police had to track down all potential alternative leads, as McNair was still rather high-profile, but on the surface, that sure spells murder/suicide to me.
As for the motive, my money is on "the girl was a whack-job".
