How are you able to open the locked door to this tomb? I tried using "remove fear" but that did not allow my thief to unlock the door.

It is unopenable until a later quest.

You will eventually recieve a quest from The Shadow Master involving this tomb. By the way it's a great side quest that has an almost comical end. -After you rock one of your advesaries to '-near death-'(hmmm...) you get a response stating "Wow that was educational!"

Enjoy