Reverse?

from the front, no matter the blocking (at least in my experience) will always be reversed, UNLESS the grabee has less than 1 full block of hp (dont know about aliens tho)

If grabbed from behind: You're dead because you don't expect it so you have no time to react. However if you are grabbed from in front (due to a heavy attack) and have enough health, you will be able to evade the execution. If you don't have enough health then you will die.

It depends on the amount of health the predator (or you if you're the grabbed one) has left.
If the grab action alone doesn't kill you, press the block buttons (LB+RB) and you break free. It's the same system for your enemies. If the grab alone doesn't drain their remaining health, there is a possibility for them to avoid the kill. (only apply when grabbing/being grabbed from the front)
The predators in the singleplayer campaign are special cases. Even if you grab them from behind, they throw you over and on the ground (if there isn't a trick to it).

I know what you mean, you're talking about in the Alien campaign when you fight the predators and when you grab them from behind they just throw to the floor over their shoulder. Only the boss predators can do this and it's impossible for the player to do. There's no way to reverse a back grapple for the player character at all, only the boss fight predators are able to do it.