The Reasoning behind The Sundering? (*6.0 Endwalker Spoilers*)

[Lyth]

I suppose it's fine to trap souls in Zodiark if they aren't from Amaurot, then.

...

This was a binary decision, with Venat's faction on one side, and the Convocation and the rest of society on the other. Do you sacrifice the lives of the other, lesser souls on the star to bring back your dead friends to resume their roles as 'stewards', or do you move forward into the future with the survivors that you have? The convocation refused to negotiate, so Venat became Hydaelyn and performed the sundering. If Elidibus is negotiating this discussion, it's going to be to one of these two viewpoints regardless.

So at the end of the day, either Elidibus convinces to Venat's faction to agree with the Convocation's plan, they sacrifice all the non-Ancients on the planet and happily live out a hedonistic lifestyle until Zodiark runs out of batteries and they all die, or Elidibus convinces the Convocation to act responsibly and listen to Venat. Although I would question how effective he'd be at any of this post-summoning, with his mind and memories in shambles. I'd also question why Elidibus arguing the case would make things any different than when Venat or her friends tried. Emet thought that he was a bore. And as the recording of Venat points out in Anamnesis Anyder,

'Yet it is plain they will not countenance a permanent solution. That being the case, we must ask ourselves a simple question: are we prepared to pursue our chosen course, even should it mean suffering the eternal condemnation of our brethren? If so, I see no further reason to demur. Let us bring forth the Light that shall ever after keep the Darkness in check.'

It doesn't matter, at the end of the day, if the Convocation could be made to come around to Venat's viewpoint. Is this the right thing to do? If so, then act.

The bit about the Eighth Umbral Calamity is a false analogy. Had they said 'Let's use Zodiark to prevent the Final Days from even happening,' then you could compare the two. But we don't know if that's in the scope of Zodiark's powers or not, nor was it ever suggested. For 'Stewards of the Star', the snippets we get of Amaurot show that it's a very insular society that isn't really all that interested in what happens outside of its own walls. Calamity affecting an advanced civilization on another continent? Why is that our issue? Volcano about to wipe out a village and some delicious polygonal grapes? Well, let's chat philosophy about it while it erupts. If it doesn't immediately impact their cozy lifestyle, they're not that concerned about it. They were all too happy to live with a temporary fix.