Yeah, so you have some more souls being rendered to him - perhaps a willingly enacted sacrifice which some nonetheless could not stomach - vs sundering the entire world, its god and its people, whether or not this kills them - it certainly erases their former existence, though, and greatly enfeebles them. Who is weighing the costs here and determining it's worth it? The sacrifices to bring about Zodiark were by all accounts voluntary and it was in the nature of these beings to sacrifice themselves for a greater cause. It sounds to me more like some other individuals could not handle more of it. That enervation is her power is of course natural given the task sought out. I agree. It's more a question of what the nature of the "problem" to be solved was and whether one deemed it worth "solving". I'm not aware of anything that says there could not be reincarnated souls, though. What's that based on?
Also, it is of little consequence whether she demands worship or not. She already has complete dominance. What does she need it for? Any Primals that arise are usually slain or draw but a slither of her aether, so she has no need for them. By the sounds of it, Zodiark did not constantly demand aether either, and any further sacrifices were to bring back those who originally gave themselves to bring about this restored world.
On that last point, I doubt they would be doing all this on a mere hunch. We see during the fight with Emet-Selch that, towards the end of his first form, he invokes his fallen ancient brethren, and then during the second he calls on the prayer of his fellow Ascians to grant him the might to see through his task.
Lastly, if Banri Oda is right, and the state of sundering has resulted in a natural state of imbalance, who's to say it is even a stable state, with or without the Ascians, as if it is not within man's nature to do evil or set about bringing imbalances? Again, the Ascians simply direct endeavours as suits their agenda, but it's not like the existing races aren't apt to engage in such activites of their own free will or that they could not, like Zenos, gain the power to pose a genuine threat even if not Ascian.
He never said that he was the person who left the Convocation. There was someone who did, which another NPC also reveals in the Hall of Rhetoric, referring to it as unprecedented. I posit that it is probably you, because he suggests as much when he says surely Emet-Selch has seen the hint of "him" in you (the character)... later, Emet-Selch gets a glimpse of you as one of the Ancients, which he thinks is delusion on his part.
His name is just a reference to a character in More's Utopia, which is where they got a lot of their inspiration for the naming conventions of areas in the city, and means dispenser of nonsense - the opposite role he's playing in the FFXIV story, funnily enough.