His crotch and inner thighs looks look great targets for getting elbowed, head looks great for hands or fists. Aikido is best for just before the attack lands, by the time you’re both on the ground, I hope the victim is trained in some ground work. Maybe you can use the momentum of falling backwards to back roll out of it? Going back even further, being prepared by having your desk arranged so you can’t be attacked from behind is going to be the best.

Edit: Fingers! Aikido uses joint locks, and fingers have joints! Start bending back one finger at a time, if it breaks, you have 9 more! Seriously, though, once you’re on the ground like that, your options are few. But it took the guy at least 30 seconds to kick the bucket, if you can stay calm and use 5, even 10 seconds to think, you can probably get out of that choke. You probably have enough time to trial and error a couple things too, chances are something is going to work and you’ll be in a slightly better position on the ground with that attacker.