A handful of half-baked reasons for the Yatwer = No-God idea:
1. TTT glossary entry on The Apocalypse includes the line "In the spring of 2143, the No-God, summoned by means unknown, first drew breath." The only other cases we've seen that involved summoning dealt with bringing agencies from the Outside into Earwa.
2. There's something fitting about the No-God being a literal inversion of an actual God.
3. Yatwer is the goddess of birth. And the No-God's existence stopped birth.
4. The Gods vs. Kellhus are an important theme in the second series. But the only unequivocal example we have of a God directly intervening in the world is through Yatwer -- why?
5. Madness flinched when he read my earlier post.
Good stuff. Some thoughts:
1. I also think it implies that something was brought from the Outside. I suspect the No-God already exists there (and has always existed), or something like that.
2. I absolutely believe that the No-God is meant to be an inversion of our stereotypical (semi-Western) notion of God. In TSA, the God is the sum of all souls, all things, the Absolute. It is the creative force behind existence, and it has all the answers. The No-God, on the other hand, is personified Nothingness (as opposed to the Everything-ness that is god). It is an inherent destroyer, a void of being/existence, and it has no answers -- not even about itself.
3. I think Yatwer's true cosmic "opposite" is Gilgaol. As we know, these two are the most popular, and the most powerful. Only they can "seize" a soul. Birth and War can also be looked at as Life and Death, which makes sense of why they're the most powerful of the gods. I think the No-God is equally destructive to both sides though. War thrives on the outflux of souls, Birth on the influx. The No-God breaks the whole system.
4. Building on my earlier thought, I would say we've seen quite a few examples of Gilgaol's presence in Earwa, though not (yet) as directly as Yatwer. There are tons of references to Gilgaol being seen in characters like Cnaiur (whom literally describes having a "second soul" at times when his rage is at maximum overdrive), or other war-like characters. The Knife of Many Hands is even more overt about this with Ratakila.