Thanks for the VERY interesting post. The guy cheering the shooting should be very easy to identify - you couldn’t get a better image of him. I assume the FBI has seen this footage; if not, they need to ASAP.
To complete my thought: the guy cheering the shooting is acting very much like an accomplice with foreknowledge. He is not afraid, is quite sure that he will not be targeted even though making himself very conspicuous, and seems to know exactly where the shot came from and is congratulating the shooter.