I've loaded the game three times, talked to Bilibump and agreed to deliver the rat pie each time, but when I open the crypt and descend, only the first door to the first chamber will open. It's empty, the second door will not open and Rethpian is no where to be found.

This is something you are going to have to get used to. All the dungeons have a trigger that loads the different area scenery. It is typical to walk into the first room, see the closed door, find the lever and swith it, the new area will load, and the doors at both ends will swap directions. When you try and exit, the crypt door will be the one that is shut. The lever in this instance is on the left hand wall about half way along - the left wall is favoured in most dungeons.

Considering that the crypt is supposed to be the first/training dungeon, it seem pretty dumb to have the lever in the one part of the wall that is plunged in utter darkness. No wonder new players have such a hard time figuring it without the walkthrough.