Since the police are standing by their report certain things would be noted.
If McNair was resting on the couch, there would be powder burns consistent with the distance from which he was shot. Blood spatter would be found in a consistent pattern outward from his body, and the blood droplets would be found on Kazemi and the gun she used.
Then, after she shot herself, the powder burns on her body would be consistent with the distance she was holding the gun, and would probably be very close to the body, indicating contact or near contact at the point of entry. Then, the blood droplets would be projected away from her body, and would overlay the powder and McNair's blood spatter on the gun and the blood droplets and gun powder on and around McNair from the injuries from the first shots.
Both bodies would be at certain stage of contemporaneous rigor mortis and blood coagulation if she shot him and then immediately shot herself. A test by the coroner's office would establish the approximate time of death.
The would be an intensely investigated crime scene, and it would be difficult in the extreme for a third party to have done it, or for it to have happened in a manner in conflict with the reports.