Not sure how, but that last chick managed to get shrink wrapped. The children profusely deny opening the incubator, so I'll consider the idea there's a cool spot, or breezy spot in the front right corner. After 18+ hours of zip without exit (gave the other three time to dry and fluff somewhat), I steamed up the restroom and closely inspected the egg.
The chick had made a pretty good job of unzipping, but then stopped. The beak had been in the same spot, tapping on nothing but membrane or air all day. The zip line was dried to the feathers and unopenable. I followed the assisted hatch page as best I could to remove the cap of the egg and to move back the dried membrane. The inner membrane was nice and clear, blood fully absorbed.
I've put chick-on-the-half-shell back in the incubator on a paper towel. S/he's mainly resting, but has managed to free both feet. I'm hoping it gets clear of the shell without further assistance. It's the only chick in the 'bator, so it will have time to rest. The belly is very round, so I think it's absorbed the yolk. It just has to get the rest of the way out and moving.