Just as an aside from the main topic - there's another way down
There's a hatch at the ocean side of the building that's pickable and leads directly down to the bottom level
I meant in the actual tomb itself. Youre next to the door, can see into the private room with the sarcophagus that holds the undying rezzer dude, the dead scribes are all over the floor having never risen and the 4 of you are bunched up at what you know is a fake wall, pressing your noses to the stone. You get a key off a scribe and then you ostensibly find the keyhole to use it in when you walk over there. I have never failed that before so we looked around for a bit and then left because its multiplayer and we quickly exhausted our low-level effort to finding an alternate, if there is one.