“no evidence so far that Brown was either shot in the back”
There is evidence. The shot that struck his right arm, inside, above the elbow.
When a person runs away - particularly a heavy-set person - their arms often “chicken wing” out some, to the side.
It would be, probably that shot fired by P. O. Wilson, that caused perp. Michael Brown to flinch, stop, and turn around to face Wilson.
Maybe. But he describes the path as “upward, backward, and leftward”, and it’s in the middle of a pretty classic series of shots for someone shooting rapidly at an approaching threat.
I don’t know what the law is in Missouri about shooting after someone fleeing — some places, some circumstances, a cop has the legal right to shoot someone who is running away. But just about everywhere I’ve looked into it, if a cop feels justifiably threatened, he has the right to shoot someone approaching him. The law doesn’t care whether Michael Brown had his arms up or not; the very fact that he was moving in Wilson’s direction and refusing to stop means Wilson had the right to shoot.
Homicide rates for “unarmed” homicide (killing with just hands, fists, feet, etc.) in the US have been dropping — they were nearly three a day a decade or so back, I think they’re down to fewer than two a day now — but they still happen, and the law takes account of that. If it’s true that multiple witnesses have testified Micheal Brown was approaching Wilson, then it’s pretty likely that the law is on Wilson’s side.