the main thing i see is that bell smacking you in the rear every time . To me that means you aren’t catching the bell with your hips, but it’s merely being yanked at the end of your “rope”. I think you need more knee flexion and to hike pass the bell back rather than letting your arms crash into your pelvis.
Yes, the bell flipping in the hole is due to a combination of not sitting back deep enough into the hinge and mistiming between the arms coming down, the hips moving back, and the torso coming forward. The hips finish early, then arms come down and crash into the pelvis, and then the torso continues folding forward. You want the arms to come down against the torso and then move
with the torso into (and back out of) the hinge. You don't want the arms hitting the pelvis or inner thighs on the way down or being pushed by the pelvis or inner thighs on the way up. They should move together, without impact or pressure between them. Also, ideally your hips should finish moving back as the torso finishes coming forward, not hips first and then torso.
Your back position actually looks pretty good to my eye, and you are not overly thrusting your hips forward and leaning back or hyperextending your spine at the top, which is also a common cause of low back discomfort with swings.
To pattern your hinge, spend some time with the touch the wall drill and the KB deadlift:
To get the feel for sitting back into the hinge, use the touch-the-wall drill:
--Stand facing away from a wall with your heels a foot or so from the wall.
--Sit back into your hinge and touch your butt to the wall.
--Move a little further away from the wall and repeat.
As you move away from the wall, if you sink your hips straight down (squat) you won't reach back to the wall. If keep your legs too straight, you also won't reach back to the wall.
To get a feel for coordinating the timing of the hips and shoulders so everything starts and finishes together at the endpoints of the standing plank and full hinge, use the KB DL:
--Do sets of 20. You can use one bell or two, and one or two arms on one bell, but do sets of 20. I found that the second 10 in a 20 rep set is where my brain and body really start to feel things and figure things out, but you have to do the first 10 to get there.
--Start with the bell(s) back between your feet toward your heels, keep the arms in tight to the body and try to lower the weight to the same spot. The bells will want to drift forward, so you will have to really sit back and keep your arms in tight. Avoid any tendency to get squatty to keep the bell back. Do it by sitting back, like in the touch-the-wall drill; your hips will move down as well as back and your knees will bend (you're not necessarily trying to keep the hips high), but keep the hips above the knees and the shins relatively vertical.
--Focus on your timing. The tendency is to start lowering the bells by sitting back as far as possible and THEN continue to fold forward and bend the knees to reach the floor. So it ends up being a two-part movement. See if you can adjust your timing so that your hips, knees, shoulders, and the bell all start and finish together. This tends to happen naturally on the positive part of the movement, so see if you can reverse engineer the timing of the positive and apply it to lowering the bell.
--Focus on your rooting. Keep your weight evenly balanced over your whole foot and try to maintain that same weight distribution throughout the range of motion. I visualize the soles of my feet extending straight down into the ground, as if I were wearing tall flat platform shoes that are sunk below the surface of the ground. In the actual ballistic swing, just focusing on maintaining even balance over the whole foot throughout the whole movement can often automatically fix a lot of timing problems.
--Get a feel for the endpoints of the lift (the zipped up standing plank and the deep hinge) and use those as targets in your swing. When I was recovering from a shoulder surgery (rotator cuff repair after injuring it playing basketball), I did a LOT of KB deadlifts because I could do them long before I could do ballistics, and I developed a new appreciation for them. I found that they were actually very valuable in helping refine my hinge pattern, even after deadlifting and doing KB ballistics for decades, and had a lot of carry over to my KB ballistics when I returned to them. By grooving the pattern at slow speed, I could naturally and smoothly transition from plank to hinge and back at ballistic speed, with everything starting and finishing together.