Note the conspicuous lack of muzzle flash from our so-called "assassin".

Have they charged him with actually shooting anyone?

Discharge of a firearm.

So he did actually fire it? At what, I wonder? Or did he just miss, like the guy shooting at him did 5 times?

From what I have read, they did find a fired cartridge in his shotgun, but they are thus far unable to determine who shot the agent.

I'd imagine that someone simply storming the security line would be the first thing security prepared for.

From what I have seen, security at the time was in the process of dismantling the checkpoint when the guy showed up. The event was under way, everyone that was going to enter was already in the dining hall. I'd imagine there were several more layers of security between that checkpoint and entry to the event space.

Given that there were only five shots fired, they must know who hit the security guy.

As I understand it the only thing that stopped him was that he tripped and fell.

Well that would certainly be an issue. Dismantling the only checkpoint would be silly in that case.

I'm fairly convinced that the guard that was shot was hit by the other guard that fired the 4 shots. Looking at the stills from the video, there are at least 7 other security guys directly in his line of fire in the background.