” It would not surprise me if these Kesachs are really hereditary Cohanim, continuing the practices of the First Temple era, despite the changes in Jewish law from which they’ve been isolated for too long. “
But - if the Cohanim exist, rabbinic authority is chalanged ...
EG, one would assume, according to the Cohanim, there were no changes is Jewish law since the first temple.
There are Cohanim in all Jewish societies, but they do not get first fruits because they are not in a state of purity sufficient to eat first fruits and truma, etc.