I need to understand the details better but if the facts are as you say they are, then yes Miller took action which assisted or abetted another in the commission of a crime, which makes him an accessory and as guilty of the crime as the person who pulled the trigger, just as the non-shooter in the car Is alleged to be.
That Bama is so wrapped up in their desire to win that Oats isn’t even going to sit Miller for a few games out of respect for the victim, and to show Miller that there must be some repercussions to terrible decisions, is forever a black mark on Nate Oats. Great coach but not a person to mold boys into men.
The defense to Miller’s actions, which I am confident he will argue, is that he had no knowledge of what was about to happen when he brought the gun, and that maybe supported by Miles’ statement to the police.