I would have busted that girl's butt, no doubt. Verbally, of course as I'm not too fond of physical violence, but I definitely would have given her a piece of my mind. Sometimes humility is the best punishment, and a stern lecture would have sufficed in this situation. Now, if it was an older person who had too much alcohol, that'd probably be a different story... But I am definitely glad your horse is okay.
As for keeping them in the trailer over night, how large is it? Do it have a divider? If you're keeping only one horse in a 2 horse trailer, perhaps you could take the divider out over-night so the horse could have room to turn around and such. Personally if I had a choice between tying and the trailer, I'd choose the roomy trailer.