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

[Nayukhuut]

That's not how the rules work. Venat tells us you need the subject to recall or you can view it etched into the Aether, albeit it is unreliable. My point being the trip may not have happened but technically the events are "preserved" according to the rules unless you want to say it was one of those events prone to waning or perhaps Kairos affected it (as operates using Aether waves). But in that case why don't the rules apply to the events in the past well? Or can Venat/ whoever only pull an instance of the Kairos event only when convenient for EW critics?

The fact is it is a flavor of Venat using the echo. It's unclear whether it's common knowledge or even how reliable it is to begin with. Venat did clue certain people into the fold and they weren't able to stop the reaction of summoning Zodiark. When faced with the choice of a tempered future she gambled and sundered the world. Very simple.

If we follow your rules and assume the aether method is unreliable, then we do have a subject that can recall what happened. Venat herself. The Convocation could have easily had someone they trust inspect her memories to see what's up. That plus the physical evidence would have been proof. I highly doubt that Venat and Azem were the only people who could use the Echo to see into the past. I suspect it just wasn't done wholesale in Ancient society because peering into ones memories is quite an invasion of privacy.

Also, Venat didn't even try to stop Zodiark. She only started really doing anything after he was summoned. She knew she needed him to hold off Meteion.