I did not crack the Da Vinci Code here. It's totally obvious that Greer fired the fatal shot.
The fake reflection causes the headshot by working in perfect sync with the fake blood mist. 312-313
In the nix film the headshot happens at the exact moment Greer's arm goes over his right shoulder which mirrors the altered zapruder film. It's that easy in a cartoon sort of way because that's exactly what the zfilm is, a cartoon of the driver shooting Kennedy.