But is there any sort of organized form of this resistance found in Golarion? Any societies, networks or other methodical groups of undead fighting the Whispering Way? Is it otherwise pockets of rivals formed from among the feeders, independent in action from the others? And after all, why would vampires and ghouls support followers of the Way, knowing how it must end if they meet their goal?
Because they're not going to end life on Golarion, and their money can be exchanged for goods and services. It was good enough reason for Ramoska Arkminos.
What do the hungry dead gain from actively opposing the Whiserping Way?
Demigods are known and referred to by titles and epithets. A demigod's worshipers generally treat its name as a sacred mystery to be jealously guarded; empyreal lords are particularly strict about this.
Rahadoum is an ex-Chelaxian crusader kingdom. The Laws of Man are less than a century old, and came after 450 years of Arodenite mission civiliatrice, Dawnflower insurgency, Hellknight counterinsurgency, Red Mantis terror, and Usij.
Stirge is a rarely-eaten local delicacy everywhere.
Also I fiddled with languages a bit:
Druidic has NO written form. (Secrecy and permanently decipherable texts don't mix.)
Hallit has TOO MANY written standards. Not many Kellids are actually illiterate, but most use scripts specific to their tribe or region and unknown outside it. That's part of how Taldane became a lingua franca.
Polyglot DOES have a written form. (The Maagambya has teaching high-Int students writing-dependent magic for 8,000 years. They've worked something out.)