As for the first point, I read through my copy so quickly I had to ask others whether Lastborn had lived or died, because I wasn't sure. I was upset when the consensus was that he died, simply because he's about the second most badass Nonman to have appeared in the actual text. I hope his pops can fill the void!
I'm not sure there's a connection between the two suicides you mention, although in both cases the characters in question seemed to have known the implications of their death (for others, or for themselves) and been willing to follow through, which for me is especially true of Koringhus -- but then the Koringhus sections were the only ones I seriously re-read.
My opinion, at this very early point, is that the qirri's effect on Koringhus was less important than it seemed. Or, rather, Koringhus had already come to all the conclusions meaningful to the reader by that point.
I'm kind of restating thoughts I had in the "Kiunnat and Zero" thread, but whatever -- I'll likely do so another two or ten times before TUC.
So, I think Koringhus was especially (if unintentionally) prepared to grasp the Absolute more-so than any before him. Well, besides Kellhus perhaps, but their circumstances differed greatly. I don't really think Koringhus was actually smarter or more powerful than his father or his grandfather. Rather, the world simply allowed him to come such a conclusion much more quickly (or did it? I'm reminded of Kellhus contemplating the twig at the beginning of TDTCB), and without the burden of being a self-proclaimed messiah.
In the Bakkerverse, the ultimate standard for "morality" (to me it's not really morality so much as the top-most hierarchical figure) is determined by an oblivious, cruelly exacting entity we can call the Zero-Goddess. It can also be thought of, I think, as Mother Earth/Earwa -- but one as viewed from the lens we might expect from this kind of fantasy world. I am almost tempted to call it a "Darwinian Goddess", given Koringhus's assessment of the wilderness into which he plunges.
I feel strongly that all of this ties into the standard Biblical notion of a Tree of Knowledge. There is no damnation without sentient beings -- at least not sentient beings on Earwa -- which is a sort of Garden of Eden, corrupted first by the Nonmen/Men (not sure which), and then even more so by the Inchoroi.
There's an important contrast between the oblivion of the No-God and the oblivion of the Zero-Goddess. The No-God is an anthropomorphization of nothingness -- which means it's not actually "nothing". True nothingness is, well, nothing. It cannot possess any other attributes, else it becomes something. And the No-God possesses many attributes.
The Zero-Goddess, on the other hand, has no attributes until other sentient beings exist to apply them. To a garden-world of non-sentient plants and animals, there's nothing to damn to because there is no one. That last bit is important -- no one.
What Koringhus realizes is the maddening simplicity of it all. Most sentient beings are damned because they deny the interval between themselves and other things -- even their children.
This is really, I think, the key to it all, and the reason we have Koringhus and his son as an example in the first place. Both denied the interval between each other -- which is a way of saying that, unlike "proper" Dunyain, they could not deny the familial connection they shared, but on a larger scale it explains the metaphysical notion of how the Absolute and Oblivion can be one and the same. By having a notion of "Self", sentient beings unwittingly acknowledge the interval between themselves.
(side note: I suspect this also why the Psukhe has no mark; its practitioners are semi-enlightened, denying the interval between themselves and nature, and thus their "sorcery" is not an aberration of reality, but a fact of it).
I'll have to add more later, likely when I've had the chance to re-read the book in full, since anything else I could theorize on would feel very shaky. I do think some riff on this concept is at play, though. It simply fits too well with the idea that "the darkness that comes before" is the key to salvation -- that is, never arising from that darkness to begin with. It also appropriately makes the audience rethink their stance on too-easily hated foes like Sranc and Skin-Spies, especially the latter.