It's possible for mortals to survive in the warp, besides the poorly written Kaldor Draigo there's also chaos champions, orkz, a silver armoured warrior who managed to face Slaanesh personally before falling, some chaos marines travel with no gellar field, the teleporting space crocodiles and various other bits and pieces here and there, so maybe a blank would just get stuck walking around the nightmarish landscape that some beings perceive the warp as.
You've basically pointed out the fundamental issue with the question: anything goes, as per usual. People have been shredded in to ethereal protoplasm by being dropped directly in to the warp, some people travel through it for split seconds, some people live in it and have grand ol' adventures. Some Chaos warfleets use Gellar fields, some use sorcery, and some apparently just bathe in raw Warp energy during their travels.
Why do Space Hulks with sporadic Gellar field coverage not emerge from the Warp as a piece of Limburger or a sound or Tuesday? Instead they emerge as mostly recognizable things with mostly traversable features wot makes for cool adventures.
The fact there's something in 40k with a stated and canon cause and effect merely means not enough authors have been given enough free reign to interpret what that means. I'm sure some author somewhere has at least thought something like "A blank being dropped in the warp creates a psychic explosion that destroys all warp energy within that part of the warp and that drives people in real space within the whole segmentum insane." I mean, Culexus? They've already weaponized that shit. Warp Storms both exist as things in the warp with a specific position and location that can be avoided, and yet the warp is nowhere and everywhere all at once, so how can any given thing be in a given place if everything is just a mental metaphor or construction to make the Warp navigable. And then the fiction attributes certain behaviors to the Warp in certain areas of space, which is a direct contradiction to the previous idea. Ships don't exist in the warp in physical space, yet they do when something dramatic needs to happen like overlapping Gellar fields. Sometimes convoys actually exist as following each other in some sort of shared travel, or a ship stalking another ship. Other times fleets reach their destination sporadically, each on its own little journey separate from the others. Entering realspace within a solar system's gravitational field creates grav waves that pancake ships and planets alike, except when it doesn't.
It's what makes answering 40k questions fun but also finger-pushing-glasses-up-nose frustrating.