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

[Lauront]

And I the only person sick of Tataru at this point? And it turns out she hasn't been keeping the Scions afloat after all, that was mammy Levellieur (and that little detail has me questioning quite a bit of what happened post-ARR).

No, you're far from alone.

Giving us more detail about the proposed final sacrifice could have strengthed her case. Venat says she and her followers didn't want to see this new life snuffed out, but what was it exactly?
If we are talking about people born after the creation of Zodiark, I can see a serious moral dilemma and I could understand people dissenting (count me in), but if she was trying to justify leaving people dead who could have been ressurected, including her friends who had sacrificed themselves, just to save a few plants and animals, then she comes across as crazy as Hermes.

The sources point towards (living) creations seeded by Zodiark, but by all indications, it was a ruse to bide time/draw support, AFAIC. Her primary (and only) motivators as given by the devs are 1) dynamis manipulation and 2) avoiding the fate of the final Dead End. She confirms 1) to Y'shtola in-game but 2) is evident in her cutscene and arguably, Anamnesis. It's telling that even after the WoL re-tells the tale of the conflict between the two sides as they know it, she still was puzzled as to why she'd become Hydaelyn and stand against her people in that way. Even if she had convinced them to stop, the problem of them being unable to easily manipulate dynamis (from her perspective) would continue to exist. So I don't think the sacrifices mattered other than at most an instrumental sense for objective 2).

Anyway, totally agree on the lack of any critical comment on how much deception she perpetrated and with the majority of your points. As to how she defeated Zodiark in spite of being much weaker than him? Why, by dragging the entire star into it, according to her post-defeat section.