The gods are in constant conflict. Therefore, if Pharasma advances towards the Mosnastery of Veils (also known as Anaphexia today), who are now Norgorber worshipers, Norgorber could also move towards Pharasma to help his newly acquired followers.
This agreement was made at the dawn of time, in a contract formulated by Asmodeus. While their followers fight with each other all the time (as seen in the cemetery and the river Styx), they are not allowed to mess with mortals in that way. The details of this contract are unclear, but it is related to Asmodeus helping the other gods imprison Rovagug.
In the same way, if Pharasma's psychopompos destroyed all the undead, we would not have any undead. But there are other gods with interest in protecting the undead, especially Urgathoa, among others. As such, there is no way for psychopomps to handle all the problems of undead or anti-Pharasmin, as they have to fight against other outsiders who want to undo their work as well.
As for the details of Anaphylaxis, you can read more about them in the Hidden Mysteries Campaign Settings book, but keep in mind that regardless of what the pharasmin kept hidden inside its vaults, it is now protected by the Anaphyxia Not only did they kill the monks and move on, they took their place keeping the secret they hid, as noted:
In this way, the Anafexia has worked for centuries, collecting secrets not only to please the use of the divine obsession of its god, but also to continue the work of the founders of Pharasmin of the monastery, preserving this wisdom in confidence for a mysterious time or an event to come. and for which the modern world is not prepared.
Keep in mind that your ranks have clerics powerful enough to cast Regenerate (cleric 7), as noted in their ability to restore the severed tongues of their members. As such, dealing with most psychopompos is a trivial matter, and the most powerful ones (such as Yamaraj) are never sent to the Material Plane.
Also keep in mind that they still pass themselves off as monks of Pharasma, their identity as worshipers of Norgorber is not publicly known.
The Norfolk killers of Anafexia pose as ascetic monks of Pharasma, and travel the world killing and stealing to accumulate a large number of secrets that only they know.