Ok, I didn't read everything in here so bare with me
A rear bare hug - Being caught in one means you are in trouble.
First and foremost you have to keep yourself on the ground, preferably going right to the ground.
There is a basic throw where you drop and twist and bring your opponent down.
You can also look for a knee bar.
Pushing his hands down and trying to break the grip can work as well.
But the key thing is staying on the ground, either by going right to the ground or breaking his posture so he can't lift you.
Don't bother trying groin grabs cause you'll already be air bourne. Headbutts won't work if he is positioned right, nor elbows or any kind of strike. He has the advantage.
I've also seen grabbing the leg and using your butt as a folcrum to take him down. This is a good basic technique but it is easily countered by someone that knows what they are doing, all they got to do is hop in and under you and throw you over top of them.
It also depends on what kind of grip they have, they should have a C grip. If they are just grabbing a wrist you can stripe there grip fairly easily.
If you are getting lifted hook there legs with yours so they can't get much height on you, hopefully you'll end up on your feet, or with both falling and going to a scramble for position on the ground.
A good source would be western wrestling, a rear bodylock is a a strong position to get and defences and attacks should be covered in any video.
Remember that there are other things which will work on someone doing it improperly / poorly. These are good if that happens. But personally I'd prefer to overestimate then underestimate. Assume they know what they are doing and if it turns out they don't it just makes it that much easier.