Never thought I'd become reference material. I'm honoured!
Seems you already have ideas for the tomb, and I don't know a lick about balancing, I'll just make a few suggestions and leave the final design to you.
I feel there should be at least one "tactical" level, small enough that someone with high perception and/or high intelligence with ESP should be able to see the entire floor. Casters in the back/rooms whose corridors loop around so you can't walk up to them easily, dark knights in the front/open room that lets them surround you. Basically, you need to plan your moves ahead or get bogged down in a slog-fest that'll get you killed. This is a holy site for worshippers of chaotic gods, it's well defended.
You could have Xinroch's Gravekeeper as a sub-boss instead of the third form (in the flesh) 'cus he's already all rotted away. Rather, I think Xinroch himself would work better as a 2-stage fight, his skeletal form then his ghost form.
Little food! Following the theme, most enemies should be necromancers/undead/dark knights, no random animals. If you're not a follower of Mellis, you better hope one of two random rooms spawn; the kitchen or the training cage with bear(s) in it.
Similarly, since it's a tomb and not a mine, I feel it should be shorter than GC. To compensate for reduced loot, more chests spawn. Perhaps chance of a single storage room with numerous chests?