So I take it as a mark of our growth that we were able to have a pretty normal ride besides all this. When she wanted to gallop off, we worked walking sideways over poles on the ground to get her focussed on my aids. When Coco and the other horse were squealing at each other, we did 10 meter circles at the cardinal points of a larger 30 meter circle and then figure eights until she chilled out and started listening. And when she freaked out over the car honking, we worked on trot-canter-trot transitions until she wanted to slow down.
And after all of this, I felt confident enough to ride her around the stable grounds and then we actually left the stable alone for the first time! She was a little hesitant but went forward when I asked her to and behaved very well aside from wanting to trot on the way back and trying to jump the paved entrance. We only went about 300 yards from the stable gate since I didn't want to push my luck, but it was a good first step.
A year ago, I probably wouldn't even have ridden her after the spookiness at the tying post. And six months ago I would only have mounted up after longeing her and then I would have waited for things to calm down before starting to trot and definitely wouldn't have cantered after the horn honking scariness. I'm really pleased that I felt so secure and in control of the situation, even with Starbuck in prancy-mode. We've come a long way, baby ;-)