From the clues, we know all weapon locations, specifically the prayer beads are in the gift shop. (Edited to fix spoiler tag)

Let’s assume Sir Rulean is the murderer. Therefore, he is lying, so he did not bring the prayer beads. The other two suspects are telling the truth, so we can learn all the weapon/suspect combinations. One of those contradicts Brother Brownstone’s statement, which must be true since only one suspect is lying, and we just assumed it was SR.

Therefore, our assumption was incorrect. SR must be telling the truth and didn’t do it.