These videos are from a couple weeks ago.
I'm so much happier with how I'm sitting. Suzanne was kind enough to watch the videos, and this was her feedback:
Much, much improved in sitting (especially in canter and last trot video). Transitions are much better, too. As a side note, turn on haunch left is good. Right she didn't have enough bend in body and also stepped back. Much less curling overall (good stretchy), but would like her a bit more off forehand. I would warm her up in rising trot, but occasionally do a few half-steps sitting with lower leg on and long to drive hind legs under. Be sure to soften reins (without leaning forward) to see if she offers a moment of self-carriage. Then forward again rising. After that, stay sitting for forward trans. Also, think medium in lateral work. There has to be more "urgency" to it. Not "oh, OK, how's this?". Try to tighten core/abs even more as if you are trying to prevent rib cage from collapsing down onto hip bones - all with chest open and shoulders back. Great progress toward that end! You still actively move in sitting, but it is more proactive (driving) than reactive (collapsing).
So since then, I'm working on a bigger trot and a prompt reaction when I ask for half steps, then another prompt reaction to forward when I give. It's going great! I love the feeling when I tighten my core and stretch up and down, tall as I can, and Clair coils up like a spring. Then I drive with my seat and give a bit with the reins, and she moves out big. And I feel like I'm staying with her! Shoulder in is getting big and floaty. Now I'm trying to play around with the same feeling in canter. The next clinic is coming up December 10-11!