I doubt there's much logic to be attributed to Venat's actions. At the end of the day, she caused her entire race to be wiped out based on her own misguided, biased judgement and because she decided to fawn over an individual who gave her a very biased account of what happened in the future.

She gambled everything on a big 'maybe' and the only reason everything worked out was due to Zodiark's continued barrier against the Final Days and the direct aid provided to the Warrior of Light by two of the three Unsundered Ascians. Which further contributes to it all becoming one needlessly contrived mess.

Then there's her hypocrisy in granting the Warrior of Light the ability to bring the Scions back to life despite criticising her own people for wanting to do the same thing. To say nothing of the fact that the 'power of friendship' trope is pushed as the reason for why the Scions and Warrior of Light succeed.

Yet a very strong friendship was established between Azem, Hythlodaeus and Emet-Selch. Assuming they would be able to get at least some of the Convocation on board with their plan to resist until the very end and work to oppose Meteion...surely they would have stood a solid chance due to their bond and shared love of the Star.

Venat herself even acknowledges that the Sundering itself was an act of cruelty and that were was no justice in the act - so I'm not convinced that it, specifically, factored into her plan directly so much as it ended up being something she misjudged as a necessity.