But... it happened. Read the Vengeful Spirit. And Alivia Sureka was not a heretic (well, the Inquisiton would say otherwise but so would they about basically everyone back during the Great Crusade) and even Big E trusted her, A LOT, since he tasked her with guarding the gate he went through to fuck over Chaos Gods so noone would do the same.
You really need to parse through the bullshit from the facts, otherwise you get things like the Khornate Knights, a small space lantern attracting the Tyranids instead of the giant blinding light of the Astronomican, and shapeshifting psykers protected by a Tzeentchian cult being treated as the Emperor's children rather than just common Tzeentch sorcerers.
As for where vail says pariahs use the warp- or even where Jurgem uses the warp, quote pls, 'cos I can't remember it and the logic ism' checking out.
Ciaphas Cain: Hero of the Imperium. Page 455. "To my relief, however, they seemed to become mildly disorientated every time they approached our hiding place, veering off before they had come within a handful of metres of us, a deliverance I could only attribute to Jurgen's peculiar qualities.
1"
"1. Perhaps correctly. The aura of terror projected by necron pariahs appears to be at least partly a psychic phenomenon, so it's quite reasonable to assume that a blank would repel them and mask the effect. However, since no other record exists of a blank coming into such close proximity to a group of pariahs, and they're far too rare and valuable to risk in deliberately testing this hypothesis, it must remain conjectural."
EDIT: Can I ask why you and Kot are both beard-wearing oranges?