It would make sense that if MS was doing the shooting, from the back seat, it would be from the right hand. Otherwise, he'd likely be catching hot brass with his face. That would require MS to be fairly close to the left passenger door, to get both the shooting angle, and the ergonomics of the wrist correct in order to do so. This is assuming that the front passenger window blow-out is due to a gun-shot from that angle. It would also explain the position of the brass as it was found on the right rear passenger seat, by the seat mounts - if it bounced off his face in a left-handed shooting scenario, i would think it would fall rather much more forward, than it did.
If MS was right-handed, and he was shooting, he'd be shooting from the right. Muscle-memory in shooting, is a very important thing. The evidential location of the GSR, still bothers me.
As always, IMHO.
ETA - as a shooter (rifle) - I actually shoot from the left, being left-handed. My right side fine motor control is quite useless, in that regard.