Seriously that quest is very easy.
When I first got in the monastery I first talked to the monk there. He told me I had to talk to the leader, but that she would not let me get to the underdark. I also saw his overhead message about how he does not trust the other monk.
Then I took my thief and started "confiscating" everything of use in the monastery, and that's when I stubled on the evidence in the room next to the head monk. I took them and showed them to her.
I never even got asked to help, I just found the evidence and solved the problem by myself.
Now, isn't that true heroism? I saved the day without aven being asked to, or getting any hints. Of course, I didn't have permission to look in all those chests, but that was for a good cause...
