Again, I think a lot of our difference in opinion is based on you seeing it as a prop gun. It obviously was not a prop gun. It was a gun being used as a prop. If it was really supposed to be a prop gun that couldn't shoot, then I think I agree with your position. But to me, a gun being used as a prop should be treated as a gun and if he knew it was a firearm that could shoot, he has some risk here.
I think the difference of opinion on this largely comes from people's backgrounds with guns. I think for most gunowners, a gun is a gun and they have trouble seeing pointing a gun at somebody and pulling the trigger as anything but reckless. But you're right that it's not a backyard scenario and that makes it different. That's why to me, the line is between pulling the trigger as part of the scene versus pulling the trigger when they weren't doing a scene (or running through a scene). If he did it as part of the directions, yes, that's the armorer's job to make sure it's safe. When he's not following the armorer's directions, he's got a gun in his hand and normal rules should apply (e.g., assume it's loaded, don't point it at anything you don't want to kill, etc).
I'm sure there is caselaw that can be applied to the case but possibly not film set specific. I suspect there will be a lot of testimony about how film sets usually operate and probably talk about other film set deaths with firearms (Brandon Lee is the only one I can think of off hand) and what precautions have been made standard after each one. If he wasn't doing anything out of the ordinary for film sets, I would not think you could get an entire jury to convict him, even if what is "ordinary for film sets" turns out to seem crazy to most gun owners.