Some green for Khov for doing the medical background. This is good for me since I don't know that much about the medical aspects of training. I rely on smart people who I trust to tell me what improves functions in the body effectively.
This is what I find really significant (though obviously you can't make claims like this without the background work that you did) because it basically asserts that there is one major factor that can be improves, and that's the ability to recover quickly and keep fighting.
From this I think we can deduce that you can technically increase the strength (or thickness) of the skull, like you can with any bone. Doing so in traditional means however could actual cause concussions.
You can't increase your resistance to concussions, and you can't strengthen the nerves behind your jaw, and calcifying/webbing of the jaw would not increase resistance of said nerves.
You can increase your ability to take a shot and stay mentally collected, which could lead you to block or evade more effectively.
That, though, can't be improved by punching yourself in the face. There is no good reason, as far as the data you presented, to believe that this is an effective training method. That's good to know.