Prokof, it is 100% possible to RNC someone with only one arm across their neck. All you ahve to do it squeeze your head to theirs, and it acts like the hand on the back of the head. Even if someone has wrist control of the arm i need to put behind their head, I just use my head to act as the arm, and it works all the time. No one expects it, because by the time they get control of the other arm, theyre letting go of it to tap out.
He cant choke you with one arm. Even if he's really long and gangly and thus can grab his shoulder with the choking arm, the arm in front of the neck is of no threat to you acting alone. The one applying the pressure is the one behind your head, as hayliks said(I'd trust Hayliks more than me), as well as the expanding of the attacker's chest. You need to get the arm behind your head out from behind your head first. After that I have no idea. Ask hayliks lol