I’d also suspect the autopsy would be crucial. If the cop, only fired one shot while Brown was allegedly attacking him, there should only be one wound with powder residue. If, OTOH, there are several wounds, some from the back and some from the front, then this witness’ story will be corroborated and the cop will be in deep trouble. One thing for sure, it’s not a Martin/Zimmerman case where all the accurate evidence did nothing but exonerate Zimmerman. At best, the whole thing is very problematic for the cop.
I do not know if this cop is guilty of any crimes. However, his life is now effectively over. There’s a 99% chance of him facing state charges and a 100% chance of facing federal charges. He’s in for a few “interesting” decades, to be sure.