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

[Lunaxia]

So in the case of the attack, we tell her what she does. In the case of Emet, we don't. Got it. I don't subscribe to your single timeline theory anyway, but you don't seem to be able to make it make sense. That's considering you're in denial that Elidibus spells out that fate can't be changed. And no I didn't take out context, he does tell you that your actions won't affect the future.

We tell her everything, but we can't tell her the exact details of how everything came to pass, because we don't know either. All she could do was try her best to set up the circumstances that would seemingly permit our future to happen, so in that respect, there was always going to be some unavoidable risk involved.

And when Elidibus says "you cannot affect change", he isn't saying we are incapable of doing so - he's saying that we can't if we want our future (the present as is) to come to pass, and to that end, for the timeline to stay as is.

Yet even should you manage to interact with others, you will be unable to effect meaningful change. For the reality you wish to save—the reality to which you must return—exists as a result of the Final Days. You cannot reshape the past to undo the tragedies of the present. Cannot unmake the sorrow and suffering fated to come.

So he's saying even if you want to help, you can't, because if you throw everything off balance by trying to change things, you risk jeopardising the current world we're going back in the past to save in the first place.

You cannot say it works just like 5.0 because we don't have access to the future of 5.0 as we did with 6.0 when we visited Elpis.

??? It is the same, we're just G'raha in this scenario. Only we want to preserve our future, whereas G'raha wanted to do the opposite and undo theirs.

Maybe this time, she was just trying to sunder Zodiark, or the local area, and not the entire world. Maybe this time she didn't want to create reflections. Or does something exist that distinguishes what the other possibilities she was accounting for here?

Nope, everything had to proceed as it did in our world's past, otherwise we wouldn't exist to be able to battle Meteion. Everything had to happen the way it originally did, including the Sundering, the shards and the Ascians going about their shenanigans with Calamities, for the outcome that she believed the most in to be able to happen.