OH! OH! IDEA!
What if, after Quote made it through the Last Cave, Curly went through it while he was fighting the Doctor? She made a stop in the prefab house, and, noticing a crack in the floor, shot it, and went in. Carefully working her way through the spikes, i.e., jumping in between them, she made it to the bottom of B1 and stopped to take a rest. During this time, Quote defeated the Undead Core, and the island started to quake and fall. Curly, noticing this, jumped up and was about to make her way through the door when a sort of biggish, mediumish rock fell on her and knocked her out.