I thought the boulder was going to turn out to be Rolling Stones.
They better explain how Johnny got the corpse. Opening the door should be relatively easy. He has the power of gravity/infinity after all. Valentine said no stand could get in, but he probably just meant by regular brute force and is not aware of every stand. Sticky Fingers should be able to get in as well. I think the main issue will be the pit the corpse is at the bottom of.