I only watched the side and back view.
1) Feedback is how you get good at knowing depth. Lots and lots of feedback. Videotape all of your worksets and look for yourself if you hit depth or not. Then adjust on the next set. Your depth is below-parallel on the ones I watched. You're probably about a half inch deep.
2) Your butt is fine. There's a tiny bit of movement, but your back stays flat. Your bigger problem is your upper back.
3) Your upper back is relaxing at the bottom of each rep, and it gets worse as the set progresses. Squeeze your upper back and trunk and stay midfoot the entire time. You're getting forward.
Make sure you're looking about 5ft in front of you. If I had to guess, I'd say you're looking a foot in front of your feet. Your triforce can't save you from excessive thoracic flexion.