"The only question now is when to pull the plug on this one."
now, and combine as the guys said.
a queenless hive cannot fix themselves t bone, so now is the time to combine what's left of the bees with another hive. the cell that appears to have 'collapsed'......it may have been dinged when manipulating/removing frames, and because of the cell's appearance, and other reasons, i wouldn't wait for any improvement, it's not going to happen.