This might help with the crossbow question:
http://www.sacoriver.net/~freegate/Crossbow%20Design.html
Based on that I would hazard a guess that the hook is set up like a seesaw with hook itself preventing the nut from moving, and a spring on the other end to keep the hook up. Pull the trigger and the hook goes down like a seesaw and you fire.
Did that make sense or is it more confusing? Keep in mind this is just me trying to envision how it would work in my mind and I could be wrong. Another possiblity might be that the hooks are aids in pulling the crossbow, but the design and the fact that they were placed with the nut makes me think otherwise.
The thing with the three shots to the back of the head is another guessing game. Could be that the blunt trama wasn't enough to actually drop this fellow right away and he was plugged from behind by one or more folk. Humans have been known to take some ridiculous amounts of damage and still keep moving for some time after a fatal blow, so this doesn't seem too out of line.
Could be that he was killed by an arrow/bolt or the blunt trama, then fell propped up against a hillside or something and the other two arrows/bolts were just stray shots that happened to hit him.
Maybe he had his back turned and got nailed in a volley of arrows/bolts, then his lifeless body got kicked in the head by a horse.
Perhaps all of this happened after the battle. He could have died from one or more of the injuries, but was shot in the back of the head by a mop-up crew who wasn't taking any chances.
The last thought that occurs is that maybe he was someone important, was captured, then shot execution style and clubbed by someone who didn't like him very much.
The fact is that we don't know, but it is an interesting mental exercise to think about how it may have "gone down," as it were.
Edit: For that matter, I'm not an expert and don't have the skull in front of me. I certainly can't tell whether any of these injuries actually occurred when the fellow was alive or on the battlefield at all. Could be that any or all of these "wounds" are actually things that happened many years later from natural effects on the body. (I rather doubt holes that match crossbow bolts magically appeared, but a skull buried underground might experience shifting of rock and dirt capable of fracturing it.)