There are 2 problems with doing a DDT in real life. First problem is that you don't end up in a very good position so unless you KTFO the opponent with it you probably should think twice about it. The second is obiously that if you grab someone in a headlock and just fall backward as snappy as possible they're not going to immediately collapse head first but instead carry your weight with their neck shortly before collapsing down slowly like when you jump into a guillotine choke. This can be mitigated in 3 different ways: 1. DDT them while they're on both knees (this isn't a common position so it doesn't really offer itself. 2. move close towards your opponent when dropping down (this will just make it a sacrificing throw and it will roll them over, not hit their head on the floor) 3. Lift them off their feet. The third one makes most sense, which is the reason you actually see a front neck chancery in MMA sometimes. It also offers the great option to move into top position.
Ahh, got it. It's not visible in the video what happens with his right hand after he went under the arm and grabbed the shoulder. Makes me wonder when it precisely becomes illegal. Could he just push with the wrist of one arm into the wrist of the other arm to add pressure as long as there is no grasping or connection going on?
It's the exact same issue as with shoulder locks (except ude garami) in Judo. Uke has to be able to escape the lock into a bad position for himself.
Brutal DDT-like head landings can happen a lot when you Khabareli people as well. Georgian grips can be quite the head traps.