Thanks, Raze, for the pointers.
This is what I've come up with. I made a backup of my save directory and deleted the world.x0 file just to see what I was up against.
Well, not only do the stairs work now, but it was then that I noticed that quite a few walls had been missing.
So my question is... How much of the game (up to this point) will be affected if I continue on with this new world.x0 file? I haven't cured the plague victims yet, or done anything in the graveyard or that shrine area yet. All I've done there is talked to a few of the NPCs, but haven't completed the quests.
Will trying to hex edit myself past the stairs be a better solution at this point, and hope the corruption will "go away" in future saves??