I do have a question though, regarding the recommendation to bring an experienced horse out with us. Things actually seemed to be going fine before another horse was out there- it was the introduction of another horse that seemed to "flip the switch." I get that it was probably being left behind by the other horse, but theoretically, it seems to me that that shouldn't be an excuse to act up. I suppose the idea is that going out with a "been there, done that" trail horse first allows us to eventually work up to being out there alone?
You've gotten some good advice here and it sounds like you did a pretty good job in a pretty tough situation.
Your horse was in your words, "up" when you took her out. The nerves started there. She was following the leader ok until another horse showed up that was heading back to her comfort zone and she kind of panicked at that point. LOTS of repetition will cure that and being ponied off another more experienced horse will help her in the beginning. I like to pony the horse alone first, and then have someone pony the horse with me on her next and then just start letting the distance get a little bigger between the horses each ride.
The other thing is the arena rider's nerves. Most arena riders get very heightened awareness outside of the arena and the horse feels the tenseness and is thinking, "If she's scared and she's the leader, what on earth do I do?" or something like that. Then you start to feed off of each other. So, if you can borrow an experienced trail horse that's really been there and done that, so you can ride and get your nerves under control it would help a lot.
The ground manners thing. This is where I learned to appreciate my Carrot Stick and rope halters. When she's trying to barge over you, letting her have it with your stick or a dressage whip while backing her up FAST will really help put an end to that. The minute she starts to go past you on the way back to the barn, she gets to go backwards as far and fast as you can go until she starts focusing on you and not the barn. You're going to be tired and disgusted before you're done and she'll be an emotional wreck until you firmly establish that YOU are in charge and set the pace and direction. The problem is, when they start getting all worked up and emotional, they can't learn and they have a very hard time listening or focusing. You're going to have to work her through that, so that you can get to the point where she doesn't feel the need to go to that place in her head. You're going to have to learn to read her really well, so you can stop the emotional flight before it gets started.
You mentioned that you worked her for 10-15 mins and still couldn't get her attention. I have worked a really emotional horse for 45 mins just to get her to look at me. Out on the trail you need to be really focused on what you want from her in order to communicate it and get through the flighty response. Figure out in your head what you will do when she starts to get silly. For instance, first I'll back her up and once she gives me one or 2 steps, I'll stop and praise her. Then I'll start over and ask for 5 steps. Once she's backing and not losing it, I'll ask her to move to the side, shoulder yield, both directions. Then I'll ask her to sidepass 3 steps. Then 5, then I'll back her 5 steps. Basically you're getting her to think about moving her feet rather than the barn and getting her focus back on you.