I agree. I hadn't really thought this through before, but grasping at the neck or belt isn't going to make any difference if it's you who is applying your weight on the belt, as opposed to someone else strangling you and you trying to remove it. A more effective or instinctive reflexive action might be to sit up again and stop leaning forwards. I can't see why he would worry about clawing at his own neck, that in itself is not going to prevent his death if he changes his mind. This line of thought has led me to wonder if there was something also preventing him from sitting up - that must have been a worry for him too if he was so worried about being able to release the belt with his hands.
Hmm, I've strayed a bit from my original question but I do wonder why he chose that method and why the jacket like that. It doesn't suggest a depressive cause on first reflection.
It makes a difference if you get a single finger in the right spot, or tear the ligature in a semi-conscious, adrenaline induced state.