Final Vale crypt...what am I doing wrong?

My problem is this: I have finished clearing out almost all of the 1st level of the last crypt, beat Myrk's Sending, but the door that leads to the upper right chamber (where the mummy resides) has not opened. I know this is how to get to the 2nd level, but the door just will not open. I have all the keys except the Mauseleum Key (don't know if that's what I need, no one seems to tell where it can be found). Please tell me what I'm doing wrong, I'm desperate! Thanks!

There are so many keys involved in this that I always get confused. And I've played through this area four times now. Very fortunately, other dungeons in the game are not like this. So don't despair.

According to the hint book, to open that northeast door you need a key from the sarcophagus at the end of the passage through the northwest door. Hope that's right. The hint book does not describe the key, though. In any case, you're still missing a key.

The mausoleum key you will need later, on the next level. This key is in a side room off the northeast passage, the one you're having trouble getting into. Look for the secret door.

no key needed (or so I believe). It's just incredibly hard to see the passage way right there. When you click on the area that you think is the passage, it is probably the wall. Try pointing your cursor below the wall and to what is actually the opposing wall and you should be able to get your party inside that corridor. Or try taking your thief and just maneuver him/her around the area where the opening should be.