YOu are really making some great improvements, and considering how little you get to ride, I want to compliment you on that. I think you are one of those people who learns to ride quite quickly and easily, and in a short time, you will have a really nice seat.
What I see is that you are rolled too far forward onto your fork. I think this has in part to do with the horse having a downhill conformation, and possible the saddle, but you end up on your pubic bone too much, with your seat bones pointing out behind you. This is a weak position. If you were out in the open, instead of on a hrose that is just following along in a school, you'd be struggling to put the horse together. So, you need to use your core muslces more to align your pelvis so those seatbones point STRAIGHT down. No butt pouching out!
SEcondly, I see you round your upper back a bit too much. This is a result of you having your hands a bit too far out in front of you. You can still have the nice following hands that you DO have if you put a bit more bend in your elbow, and connect your arm to your body , via a bit of a feel at the armpit area. What I mean, is you kind of anchor your upper arm a bit to the ribcage, way up near your armpit. this adds strength and stability.
YOur right heel is come up, and I think that's part of the rolling forward on your fork, makes you grab up a tiny bit more with the knees, thus the heel comes up.
This used to be a huge problem for me, and invariably, the right heel. Now, it's just a large problem, instead of huge.
Sorry to see that the other horse really looks lame.