You're all over the place with this.
He does NOT regain possession. He regains control. There is a difference, and to understand what happened, you need to stop mixing things up.
He lost possession when the ball came loose. He must regain possession. Control is one part of possession, but not the only part.
Again, you're missing the mark. He can't maintain possession because he has LOST POSSESSION. He must regain possession first before he can maintain it. There's nothing to maintain.
If he had possession, yes, he can switch hands. If he had possession, all he has to do is break the plane of the end zone with any part of the ball while active in the field of play, including the pylon extended.
But again, he does NOT HAVE POSSESSION. Let me repeat: HE DOES NOT HAVE POSSESSION.
This is going to turn into my Dennis Green moment. IF YOU WANT TO CROWN HIS ASS, THEN CROWN IT *pounds laptop with fist*
Sooooooooo...
To regain possession, he's basically under the same rules as completing a catch.
He needs to do the following, which means ALL of the following. You keep pointing out things he's done while ignoring the one he hasn't. This isn't horseshoes, close doesn't count, he needs to check every single box.
1. Establish control (check)
2. Touch the ground inbounds (check...temporarily)
3. Maintain control of the ball long enough to establish possession (not really)
In addition to those 3 things, because he is going to the ground, he must ALSO:
4. he must maintain control of the ball until after his initial contact with the ground, whether in the field of play or the end zone. If he loses control of the ball, and the ball touches the ground before he regains control, the pass is incomplete. If he regains control prior to the ball touching the ground, the pass is complete. (no, because)
In addition to that, he is near the sideline, he must ALSO:
5. If a player goes to the ground out-of-bounds (with or without contact by an opponent) in the process of making a catch at the sideline, he must maintain complete and continuous control of the ball until after his initial contact with the ground, or the pass is incomplete. (not even close).
He must do all 5 of those things. ONLY if he does ALL 5 will it be a touchdown.
So back to your post.
The ball never contacts the ground, so if this had happened in the middle of the field and he doesn't have control and he rolls and the ball bounces around a bit but doesn't touch the ground, that's fine, it's a touchdown.
But because he's near the sideline, this is not about if he maintains control when hitting the ground and rolling over. The criteria is COMPLETE AND CONTINOUS CONTROL OF THE BALL until after his initial contact with the ground. That's why that ball can't move at all. He does not regain possession inbounds. It never happens.
You keep pointing to one or two items on the list. This isn't a pick and choose thing. Due to the circumstances, he must check EVERY.SINGLE.BOX. He does not.
Source:
Completing a Catch | NFL Football Operations