Rather, I would say it's almost impossible.
"This problem was non-existent or more easily controllable in hunter-gather bands, composed of about one hundred people, where such small determined groups could easily be noticed and squashed; possibly there were no incentives in small hunter-gatherer bands to form tightly coordinated priestly groups"
The density is less than 1 in 100, so it would be real unlucky for a band to have even two fanatics. You can't form a coordinated group when you can't even get a third conspirator. You and your best bud can't pretend to be a crowd with a consensus no matter how loudly you posture. It always comes off as a Shakespearean comedy relief duo, with a stooge and his crony.
Further either you would have grown up with them, thus thoroughly familiar with their twisted fanatic tendencies, or else they would be outsiders. Many tribes swapped members like ape bands - either sending the men or the women to go foster out at the age of majority, for outmarriage purposes. You wouldn't trust someone like that, though. The difference compared to folk you grew up with or who watched you grow up would be like night and day. Outsiders would have to earn every shred of respect, and the fanatics would simply be unable to earn it.
Finally even if a band was unlucky enough to get seized, it would die to external competition in short order. Literally, in those times. The next band over would notice they stopped being able to hunt so good, the spears would come out, and then the ideologically captured band's women would all get to be someone's second wife, whether they liked it or not.
By contrast, even if you have say 15 people, you can get converts by holding a party of about 20. Their conformism instincts see that "most" people around them believe whatever nonsense, and they reflexively start believing it too. Mimic a global advantage with a local advantage, parley that into a slightly less local advantage, repeat as necessary. Only not guaranteed to work if there's a competing fanatic/psychopath group doing the same thing.
I doubt the tribal instincts are even designed to count up to 100. The only way you would get a dozen-man subset in a tribe doing the same thing is if everyone would do that thing, for too many reasons to list. Asch conformity works when "most" is four people, so there's that.
P.S. As a result of the strategic superiority of psychopath-fanatics in homo hypocritus gullibilis, is it now the case that popularity is a negative indicator, epistemically speaking. When testing an idea, always test the minority ideas first, because you'll find the correct one much faster that way, on average.