Walkthrough questions.

Well, it says there that "If the switch above the buttons can be moved, then you have found the correct combination." I don't know what it depends on, but it didn't move for me either, even following their guide. I just fiddled with it, pressed it a few times, and it worked.

P.S. Invisibility... I turned it into a race to see who would finish faster, me or the monsters :) I was playing with a party of paladins and just doing the quest for the sake of it.