More riding questions for yall!

Is she "pinning" pinning her ears or is she really just turning her ears back to find out if there's a verbal cue coming?

This might still be a pain issue. Tripping is often a sign of saddle fit problems, and the short striding, pinning ears, and offering to buck are pretty classic signs. I would definitely get the chiropractor out, I think that is a good first step.

I agree with Maigray. Sounds a lot like a saddle fit and pain issue. Especially if she was good at the trainer and is acting sour back at home again.

Hmm, I don't know how to tell if the saddle is bothering her. She's extremely heavy on the forehand, which is what I associated the tripping with, but I'm fairly green myself.
How do you make sure the saddle is sitting properly? I know how to see if it's leaning forward or back, but not if it's pinching, etc.

You know what I am going to say....she is young, she is unfit, she is green, you are asking far too much of her, and you are now thinking of punishing her (however mildly) for saying "I am young, I am green, I am unfit and I do not understand what you want of me"
What part of this makes sense?

The easiest way to check saddle fit is to place the saddle on her back without a pad. Then run your hands up under the saddle on both sides. There should be even pressure over the entire back. Any areas where it is tighter would indicate a pressure point. You also want to make sure that it has good clearance through the whole gullet and that the back of the saddle doesn't dig in. The most common locations that I find pressure points are the shoulders/withers and the back edge of the saddle.

The chiro came out today!!! He popped all sorts of things! He popped her neck and she was in heaven, just licking and chewing and falling asleep on us she was so relaxed. Then he got to her back, he said her vertebrae were twisted a bit, and he worked out all the kinks there. He said that she wasn't sore at all in the upper parts of her back (near the whither) and that she didn't have any symptoms he normally saw associated with poor fitting tack. He got to her hips and had a lot of work to do! It totally explains why she was doing everything she was doing. She had been dragging her back right a bit and really really short strided on her back left.
Anyway her hips were rotated! Her left was much higher than her right. He moved her hip around and let me feel how it was all connected, all the muscles in her lower back. It was so cool. Anyway, he moved the left hip down and the right hip up. Her left stifle was also off because of her hip. He said her shoulders were great, and he used some laser thingy that helped open up blood flow to the areas he worked on.
He's an actual human chiropractor, which is kinda cool. He went to the same highschool and college as me, too!
Anyway, I walked Jazz a bunch around the yard and let her graze some actual grass, and tomorrow I'll lightly lunge her and maybe ride, but not more than 15 minutes or so. Then I'm going out of town so she'll have some time off!

I rest my case!
Well done, though, for getting to the bottom of it, I am sure you will notice the difference once she has some muscle in the right places.