What's the point with that quite lackluster story in Endwalker?

[AziraSyuren]

Only the Convocation was tempered, as a side-effect of the summoning due to the power that went into it, and even then, Emet-Selch could deviate from his original plan, where he deemed this to be appropriate. Elidibus was controlling the primal and emerged from him to mediate the dispute, so we can reasonably infer he knew what was happening with the souls inside of Zodiark, and may have even brought this into discussions between the ancients on that topic.

As for exchanging the problem? I don't think that'd necessarily be the case. The setting doesn't outright say it, but there does seem to be a differentiation in the souls it sticks in sapient races, like the ancients (to the point that even after thousands of years, we can reliably say the sundered are fragmented ancient souls), and those that go into animals and non-sapient monstrosities, which wouldn't have cultivated the notion of returning to the star as a yearning. So as an exchange of power so that the primal remains at full functionality, to liberate the souls of their brethren, it makes sense.

We need to recall Venat did not share the specifics of her concerns with them.

No offense but that's about as much of a reach as the excuses people make to defend Venat. It was equally implied that the newly sown life could've inherited the star too, if I'm remembering right, which is something that probably wouldn't have been said of non-sapient beings. Some might've thought that they were non-sapient, but I think the fact that there was such a division in Ancient society over whether or not the third sacrifice was justified points to the opposite. The people who had problems with how they dealt with normal Creations were in the vast, vast, vast minority, and that was because Hermes thought they were sapient to some degree.

Also, I don't think deviating from the plan would've included doing away with Zodiark, and the only people who reasonably had the power to stop him were the Convocation (and ex-members, i.e. Venat, but then we'd be right back where we started). But it's definitely possible that that's not the case and they would've been both willing and able to unsummon Zodiark after dealing with the source of the Final Days.

Upon further consideration, I do think that Elidibus knew about the nature of the souls inside Zodiark, though, as it was heavily hinted if not outright stated by him before we went to Elpis. Which makes it interesting that he detached himself from Zodiark to mediate the division rather than convince people to go ahead with it.

Are you talking about the 3rd sacrifice? Where are you getting half? Afaik the only reference we get to that is Hythlodaeus saying they’d sacrifice a portion of the new life. Not half.

It might not be half, but the specific amount doesn't matter and it's just splitting hairs at this point.