Sound of the trap was it breaking not locking. They showed it later up close broken. That sounds a bit farfetched for a fairly human, not mythical, werewolf, but hey!
Whether or not the driver of the caterpillar hears the sheriff, he certainly can see the sheriff's posse chasing someone or something and could block the bridge on his own.
Though there are several times the posse has clear shots and shouldn't miss, running and shooting and hitting a target is damn hard. I've tried it! Sometimes you don't take the shot because you can't steady up, sometimes you take the shot and miss. And hitting a running target, human or animal, is harder than you think it would be.
Last line..."He's going home." His suffering is over, he's going home to be buried. Never took it any other way.
I thought the same thing about the last line, although I did think the ending was a bit sudden. Still, it would have just dragged out if there was a scene where the Sheriff broke the news to the wife and son and had some wisenheimer explained all what happened.
It is a good monster movie.