Disappointed in one Endwalker Aspect [SPOILERS]

[Kordarion]

And yet the argument against helping is predicated on the idea that it is impossible, when that isn't even supposed anywhere in the story.

G'raha's time travel is NOT proof of multi-timeline travel, he DID NOT TRAVEL BETWEEN BRANCHES, instead he traveled back to the past of his OWN branch and that was the catalyst for the branching timelines.
G'raha time travels to change the past = creation of the branch timeline.

It is true we regularly do impossible things , and that our characters don't like to give up on saving people if they can help it but can we really? Based on the fact that this is how our characters are framed by the writers maybe, as an entirely meta reason, it is impossible because the whole point of endwalker is to move on from the ancients and the past and to tell new stories beyond them. So it stands to reason then that it would be impossible to cross branch timelines not because of an in game reason but because the writers want to move on from the unsundered world and wouldn't be able to if they had you save it.

We don't have any proof of Venat taking action i'll admit that, there is no proof she did anything. But the absence of proof is not proof of her doing nothing. For me it makes sense for her to still have acted considering that during our goodbye in Elpis she mentions that she has plans to make a group separate from the convocation to deal with the upcoming crisis. Admittedly this could be her moving straight onto plan sundering but we don't know and as I will say again and again lack of knowledge of her actions does not mean she didn't do anything.

Also do you remember at what point she says her people will never change, I do. It happens AFTER the final days, after they have summoned Zodiark and refused to move forward from their tragedy, not before but after. She didn't give up on her people changing until after her supposed inactivity. That I believe counters your point of her inaction due to giving up on her people.