Being repeatedly hit in the head at full force is not good. Most sparring is done at considerably less than full strength for that very reason. I think MMA fighters take less of a beating to the head than boxers because the fight is more varied, and seemingly paradoxically, since the gloves are smaller, if a solid punch gets through, you're more likely to get knocked out. Boxers, with the bigger gloves, take more a continual beating to the head.

If you go out from a chokehold, it's either because your training partner is an asshole and didn't let go (unlikely in my experience), or you were stubborn and refused to tap (more likely). The seizing is typical when coming back from a blood choke - not good, of course, but it's what happens.