Yeah, if you use some extra type of scanner like a detector or a spy you need to cut back on the normal scanners. Having seer, priest and detector in the role list but only using two of them is what I would have done. Adds a bit of uncertainty to the setup which is good without messing too much with balance. (Similar to havinging GA, doc and padre in the rules but only using two of them)Note of caution:
In a big JL - where all the big players have flocked to the mouthpiece, the protectors, the seer and the priest - the detector is incredibly powerful. In this game, you didn't know it at the time, with no idea there was no GA or apprentice, but every person the detector scanned to be a non-villager had to be a baddie (correct me if I'm wrong). In a game where the JL can get every apprentice and important goodie to them, the detector has the possibility of being a seer and priest rolled into one.
I like the idea, it worked well here and I'd like to see it again. But to any future GM's thinking of using it, be aware it can be a lot more powerful than it would seem, and think carefully about how you'd balance it.
For the baddie side actually having either a sorcerer or a detector but having both in the rules is what I would have done.