Me espousing a possible solution that happens to take place before the events of Elpis is more of a convenience than me agreeing it is the only way. Within the realm of fiction and headcanons, you can devise a solution really at any given point inside of the timeline prior to the Sundering. I chose prior to those events to maximize potential survivors and to minimize the amount of extra baggage the WoL would require to effectively deal with the problems faced after the events where Hermes orders the Metia to destroy the Universe. Nothing is impossible and defying fate is something Final Fantasy is all about, but also minimizing the number of McGuffins and Deus Exes is also something required to make the story more believable.
Venat's plan also hinges on putting all her chips onto the WoL. All events post Elpis would be out of Venat's ability to predict due to the WoL not knowing what would happen upon returning to the present, thus unable to communicate this. As such, the following things would be unknown to her:
-The ability for the Sharlayan's and the Loporrits to construct a ship that is capable of FTL travel and is able to reach the absolute furthest point away from Etheirys in the Universe, aka Ultima Thule (note: She directed the Sharlayans to build such a vessel and it is possible she knew about what it had to be able to do since she had a Meteion tracker, however this is not explained and thus the former statement is allowed to remain.)
-The WoL being able to overcome the Endsinger inside of its own domain, which was not suitable for life and required the sacrifice of multiple Scions to reach Meteion. (note: She did enhance the Azem Crystal, but whether it would be enough would be something she would not know.)
-Zenos being pivotal to her plan's success without knowledge aforethought of him needing to be present in the final battle.
-Any and all events occurring outside of what the WoL would know she would have to direct in a way to get to all the events the WoL is describing as events as part of the MSQ (all non-essential quests outside of the MSQ can be ignored since this will differ from WoL to WoL).
-The timeline G'raha Tia was in had to play out exactly the way that it did, and her influence there would be extremely limited if any at all.
This does not include the issues surrounding the paradoxical nature of the Sundering, as there is no indication of whose idea the Sundering was. It cannot be any incarnation of the WoL in a timeline system that does not have alternate universes, as their existence hinges on the Sundering being a thing so there would never be a sundering ever unless the idea originated beyond them. In a system where Alternate Universes are allowed and time travelling spawns alternate universes, only the initial timeline and potentially the second one would be affected since the idea still has to come from somewhere, but the existence of the WoL inside of the past can be explained away a bit more simply from a logical perspective.
Yet despite all these things, the plot still works out in Venat's favor. Defying fate, defying destiny, and defying reality are all hallmarks of a Final Fantasy game. Boiling down everything into something that plays out like dominos or even a butterfly effect is effectively useless here, as the game plays out in a realm of fiction. Yet, you would likely rule out such a plot from being viable years ago with the logic you are running with for my ideas.
It is clear you are applying deterministic thinking to a game franchise that largely goes against this notion in favor of overcoming impossible odds, such as saving the Ancients from themselves. The WoL saving the Ancients from what appears to be a predestined fate from our current perspective would make for a Final Fantasy franchise styled plot as the WoL goes up against a seemingly impossible outcome, only to defy it to save those who came before, and whose sacrifices now result in their initial goal: their salvation.
So once again, I pose to you this question again: Why are you playing a game/game series where determinism is one of the least supported concepts in its themes, and is usually torn to shreds by the end of the game?
As always, when I ask these questions, it is not meant to be taken as an offense. It is me making an observation I would like to have an answer to. And while you are not obligated to answer the question, it would help me be able to potentially come up with something to your liking.