I did not have to leave as I previously thought, so here is my somewhat ordered thoughts on the whole series:
As far as I can tell, Eärwa/Eänna is located on the crust between the “earthly” universe and “the outside”, the realm of demons. The reader presumes the place that Eärwa/Eänna is on is a planet due to its similarity to earth and its inhabitants similarity to humanity during the middle-ages, but there is no real evidence as to the nature of the world itself. However, during the events at the bottom of Cil-Aujas we see that the hell is literally “beneath the feet”, as is the description of the Plains of Mengedda during TWP. This also mirrors classical christian theology where human existence is a place wedged between Hell below and heaven above, and the whole series is drenched in classical christian theology. Ironically this also means that the erratics who cower beneath Ishterebinth trying to hide themselves from the outside are closer to it than anyone else.
We also do not know how large the “earthly” universe enclosed by the outside is. The Inchoroi appear to have eradicated every other ensouled life inside the universe and Eärwa/Eänna is the last place, which is why I think the ark crashed. It was no longer needed and the destruction the impact wrought would only serve as a means to accelerate the cleaning process.
I have also come to the conclusion that the Fanim were the true, correct faith that deduced the true nature of the universe. My evidence for this is that their sorcery does not bear the mark. I think Bakker illustrates this way of human folly and its “might makes right” approach. The Men of the Tusk didn’t win back Shimeh because their theology is correct and The God marched with them, but simply because they were stronger. Who is actually right is immaterial.
I also think that the Fanim are absolutely correct about The Hundred. They are demons from the outside, slightly more sophisticated than the common ciphrang but not really that different. They also feed on human souls and they only pose to be worshiped in order to mark their morsels for themselves. This is also reflected in the discussions between Kellhus and Proyas in TGO. Yatwer doesn’t hunt Kellhus because he is a false prophet, but because fewer people worship her and thus she gets less souls and less tastier souls. As discussed above, the difference between being saved and damned isn’t where you go, but who eats your soul when you get there.
But where do souls come from? Souls simply split off and are transferred during conception or gestation inside human beings. Souls beget other souls inside this universe, which is why creatures generated by the Tekne are soulless. Both the Inchoroi and the Non-men are also humans, but are simply altered over long time. The Non-men separated themselves from the human tribes of Eänna and followed an evolutionary path to the point where they thought themselves distinct from humans. The Inchoroi altered themselves with the Tekne. However, both races still have souls, as do humans and thus they both fear damnation.
One thing I don’t really have a good explanation for is the deal with Sorcery and the Chorae. Sorcery appears to be the ability to alter the state of atoms via thought expressed by words, or simply by the soul. Three of the most common spells are making things glow, making things ignite, and rearranging air to form a lens. Doing so imparts a “mark” to both the object altered and the soul of the person who made the alteration. They are designated a morsel for the lowest of the lowest of ciphrang. The Chorae on the other hand both protect one from the affects of sorcery, and kill any sorcerer they touch by turning them into a pillar of salt (an allusion to Lot’s wife in the bible, I believe). But why?
Chorae are described as iron spheres inscribed with runes. My current theory is that each Chorae enclose a piece of The Outside inside them. The runes are a mechanism that opens and closes the iron sphere that reacts to the mark in a certain vicinity around the sphere. If it detects a mark the iron sphere opens and all sorcery is absorbed into the outside. The salting happens because this is what happens to material of the earthly universe when it comes into contact with the outside. This explains why sorcerers get slightly salted when a chorae goes by them without touching them and why the few perceive chorae as holes in existence. The hole in this theory is Kellhus having wandered the outside though I have two potential explanations for that: Either the Deimos is a sorcerous technique allowing in advanced stages to shield oneself from this salting by the outside or simply allows for only the soul to enter the outside without the body coming in contact with the outside, or Ajokli taught him a trick to avoid being salted when in the outside.