What calamity? Once zodiark is summoned and shield ethyris, they're fine. Even after sundered for 12k years the barrier still hold fine. And they manage to be solve their own challenges and problems just fine before zodiark, so why would they need him for something trivial? The Ancients were sad by the fact that the sacrificed souls could not return to the star, so much so that they intended to sacrifice new lifes in order to free them. It's not a simple "oh no, my concepts isn't working as intended, better summon lord zodiark to fix it lol".
That's a possibility, true. Just fyi, what I meant by first explanation is the writer intentionally make things vague to keep us in the dark.
But all of those accusation you made about how "the Ancients will keep using zodiark" is just pure headcanon and hypothesis. You said it's weird to assume elidibus act by only single line.... But at least I have one 'proof' about it, unlike your point. That's what people have been telling you this whole time. You don't have proof that they will keep sacrificing life for any conflicts, problems, or issues that may arise after the Final Days. The Ancients aren't some hivemind incapable of thinking outside of box. They have debate halls to argue. They intentionally have Azem seat whose main job is to help the rest of the ethyris outside of Amaurot. It's just the final days blindsided them so much they were in a shock and made poor decision (third sacrifice).
*because someone doesn't even bother to tell them.
Edit: also want to add that the magic thing is just weird argument. The world of darkness happened because igeyorhm is too gung ho in her attempt to make it dark aspected. Just having "dark" spells won't do anything to the world. Besides, that would require them to constantly overuse it and we know the Ancients very rarely goes into combat. Creation magic is different from the usual magic attacks.