Once you open all 4 of the central tombs on the raised platforms (with a strong monster and a few skeletons appearing for each), the 2 central tombs will be open. The order does not matter for the 4 tombs, and with the two central ones you can only open one, the other will be locked; they both lead to the same spot on the next level, however.
When you cure Mardaneus by clearing the catacombs, the next time you enter George's shop there should be a cut scene. I am pretty sure you should still get this, even though he is gone before you got to that point.
