There is no official word.

About chapter 15:
The tunnels under Kyudea and Ishuäl are referred to as similar in architecture, and then as Nonman work.
For Kyudea it is confirmed to be Nonman architecture. I can't remember: Do we know this to be the case for Ishuäl as well?

We don't know it for sure, but I think it is fair the read it in, since Ishuäl is an Ihrimsû name, meaning "Exalted Grotto."  Seems likely to me that the Kûniüric High Kings learned of it somehow, likely after it had ben abandoned by the Nonmen and then used it for their own purposes.  Meaning, of course, that the Thousand Thousand Halls were carved with some Nonman purpose, the Dûnyain only later found their own use for them.

I realize that I tend to drop what are essentially non-sequiturs here, but as a disorganized mind as mine is, that is the best I can do at the moment.  I came across this though:

But in the second place, “the concept does not only have being within itself implicitly – it is not merely that we have this insight but that the concept is also being explicitly. It sublates its subjectivity itself and objectifies itself. Human beings realize their purposes, i.e., what was at first only ideal is stripped of its one-sidedness and thereby made into a subsisting being. … When we look closely at the nature of the concept, we see that its identity with being is no longer a presupposition but a result. What happens is that the concept objectifies itself, makes itself reality and thus becomes the truth, the unity of subject and object” (LPR 3:356). The concept, like the human “I,” is alive and active; its activity can be called a drive, and every satisfaction of a drive is a sublation of the subjective and a positing of the objective (LPR 1:438–439)

LPR refers to Hegels Lectures on the Philosophy of Religion.  Of course, I am linking this, in my mind, to what the Dunsult tell us about how the No-God is, to them, the Absolute, a unity of Subject and Object.  In Hegel's terms, this seems to mean it would be Pure Being, which might be a hint as to why it invalidates the Outside and so sin.  That is, in Pure, Immediate Being, meaning is also Immediate.  There is no mediating term of an Eternal perspective.  Everything simply is what it is, there is never any true Becoming, it is all just Material doing whatever it is that Material does.

I'm not a slow reader but still it took a while to get through TWP again due to life getting in the way 

Chapter 6 had some funky references during the Battle of Mengedda:

Now the wind came from the east, and men swore they could smell the sea.

Mengedda being quite far inland, it gave me a flashbacks to the Battle of Dagliash, though that could be because of the "Saubon dying" passage in the same chapter.

And when the day is done,
In our eyes the Gods shall lurk!

These two lines of the chant that the Inrithi sing during the battle struck me as a weird parallel to the "What do you see?" passages that we get from Akka's dreams by the thousands of sranc. The parallel being "After we have killed, the (No-)God will be able to see what we have done (and reward us?)".

Another passage that especially struck me was the one in chapter 17. where Kellhus finally breaks Esmenet, using a repetition of words that we encounter later in TGO in the passage of Koringhus's "cuts and cuts and cuts" revelation:

"You break and remake, cut and cut and cut, all so you might answer in you conqueror's tongue!"
[...]
"And you tell yourself", Kellhus continued, "'These tracks I will not follow!', Perhaps you refuse certain perversity. You pretend to scruple, to discriminate, though the world has forced you onto trackless ground."
[...]
"'What love lies beyond sacrifice?'"

Though worded slightly different, I encountered the same explanation a few chapters later, when Cnaïur finally understands how Moënghus and Kellhus have controlled him all along:

He was bound to the Dûnyain as the Dûnyain was bound to Serwë's corpse - bound by the cutting ropes of an unconquerable hate.
Any shame. Any indignity. He would bear any injury, commit any atrocity, to whet his vengeance. He would see the whole world burn before he would surrender his hate. Hate!
[...]
Hatred, and hatred alone, had kept him sane.

Just like Esmenet, Cnaiür has been chipping/ cutting away at himself to fit his view on the world, and to soothe his own mind. In the case of Cnaiür, these cuts seem to be both physical and mental, with him scarring himself with his swazond to pove the point of his hatred... Which in turn gives an extra dimension to Mimara's Judging Eye vision about him later in TGO. I'm currently under the impression that, especially given how we see Kellhus's deduction at work, the Dûyain see the cuts that people made in reverse, since we get a similar deduction described again when Koringhus has his Zero-God revelation.

With the knowledge of the description of Saubon's death in TGO, there is the fun passage in TWP where he calls for his own damnation:

[Saubon:] "Then fie on it! Fie on the truth!"
[Kellhus:] "And what of your immortal soul?"
"Then let it be damned!" he roared, leaping to his feet. "I embrace it - embace it all! Damnation in this life! Damnation in all others! Torment heaped upon torment! I would bear all to be King for a day! I would see you broken and blooded if that meant I could own this throne! I would see the God's own eyes plucked out!

Finally, chapter 23 has the passage where Kellhus has his revelation:

And upon it two silhouettes, black against clouds of stars, impossibly bright.
The figure of a man seated, shoulders crouched like an ape, legs crossed like a priest.

I remember a similar passage (in TGO, I think? Or maybe in TWLW) where Kellhus encounters people with animal features. Did we get an explanation about that somewhere, or is it simply his madness that is leaking through?

I know there are supposed to be parallels between TPN and TAE, and I'm still amazed at how Bakker pulled it off  .

Going to pick up TTT again next of course, and I'll share the things that stood out to me, with the knowledge of how TAE ends.

Yeah, I mean it's ambiguous.  In fact, I think I want to revise my position.  It probably makes a bit more sense to me now if Wutteät is another Ark-made thing.  Post-Fall though they don't have Ark to guide the development and can only work from the existing Wutteät pattern.  I'd still guess Wracu have souls though, even as derivative as likely they are.