For the "Stones", you may want to restrict the number of "guesses" the owner can make so that you don't have to moderate the whole spamming-guesses thing.
Examples:
Version 1: Super friendly
Mysterious Stone: Owner may attempt to guess the identity of the Wizard or Apprentice once per night. "Pyure is the Wizard!" -> "Nope"
(Owner is a super-weaksauce seer in this case)
Version 2: Mysterious
Mysterious Stone: Owner may attempt to declare the identity of the Wizard or Apprentice to the GM at any time. If he declares incorrectly, he dies.
(Bit harsh but it requires the owner to FIND the Wizard/Apprentice, not just guess)
Version 3: Compromise
Mysterious Stone: Owner may attempt to declare the identity of the Wizard or Apprentice to the GM at any time. If he declares incorrectly, he may not guess again for three days.
(Now you pay a penalty but its not as harsh)