Keep working on the trumpet Jimmy, it can be a rather fun instrument to play.
As for how to hold it, your right hand was correct, but your left hand wasn't. For your left hand, your thumb goes above the first valve slide, and the rest of your fingers go on the opposite side of the valve with on...
And about removing tired cops from the scene. They simply couldn't do it. They were undermanned and working with what they had in a situation that they weren't fully prepared for.
Keep in mind that, although a lot of the protests were peaceful, there were a bunch that were quite violent and as the video showed did a bit of damage to city property.
Also the cops were almost always outnumbered 50 or more to 1 and were working 12 hour shifts. The fact that they are using tea...
The face at 1:13 makes me think "Bueller"