In spite of everything else, thanks for sharing this. It's nice to get some fresh, genuine lore behind the Ancients for a change - and moreso when you get that happy little pang of satisfaction when one of your personal headcanons pans out! Right from when 5.0 dropped, I always saw Azem's defection as an out-and-out condemnation of the initial Summoning and the sacrifices involved, and it's really cool to get that confirmed. Even without reading between the lines of Emet's behaviour, it seemed like such an obvious WoL thing to do... though it makes subsequent events truly all the more frustrating. They really did let go of a lot of what was innate to the WoL as a character to make Endwalker work, and that'll always be a huge disappointment for me. Ah, well.
That pre-existing creation magicks began warping first is also interesting, and paints a much better picture of why they must have been overwhelmed so quickly when they had a greater degree of control over creating monstrosities than mortals did/ do. The keening noise to me seems like one of those plot points that existed pre-Meteion that they've tried to rope in to the main story since, although it's not the clumsiest retcon they've done. I can never shake the feeling that something was actually intended to be going on with the lifestream initially, but that's that, I suppose.
And the point about the schism just makes me... sad, really. Imagining the panic, the fear, the insecurity, and everything unravelling as everyone struggled to decide what to do; the way it broke not only their people but the bond between them. It was such a tragedy, and it's unfortunate how often that gets skated over in favour of the "the Ancients were bad/ cruel/ callous and had it coming to 'em!" narrative. Amidst the hailstorm of Garden of Eden metaphors and contrived Buddhism philosophical referencesand blatant fanservicethey lost so much of the humanity they'd previously been permitted as characters, and it's one of the biggest pitfalls of EW's writing, to be honest.
Also, (not as a jab to anyone but in general) the Nibirun inference will never be the justification the writers clearly hope it to be.