Venat successfully tagged Meteion and had the coordinates of her nest. She was also a well respected member of her society so the idea that the Convocation wouldn't listen to her is highly flawed - particularly if she came to them before the Final Days occurred. In addition, she had access to the Echo and we even see her using it to show the Warrior of Light what Hermes did with Meteion. Then, of course, there's the simple fact that many researchers throughout Elpis would have seen Azem's familiar travelling with Emet, Hyth and Venat. So Kairos ridding Hyth and Emet of their memories is largely irrelevant.

Zodiark was an essential part of her plan, so her opposition to summoning him was always feigned and exploited as a convenient excuse to misdirect her own followers (who she never revealed the truth to, either...)

Furthermore, the first two sacrifices were entirely willing and born of a plan painstakingly forged after other alternatives were pursued. A necessity to return Etheirys to a state of habitation.

Ultimately the game established that the protagonists belief genocide, eugenics and racial replacement are lines in the sand never to be crossed no matter how sympathetic the motives behind them.

She dithered on how the ancients would react to the report, in Ktisis saying most would simply be content to continue as they were with this knowledge, other than Hermes of course, then resorting to the convenient nonsense about the masses panicking later on. Even if that were a possibility, there is nothing stopping her from just telling the Convocation and handling the matter discretely with their aid. Besides, I'm not sure what this is about the ancients' attitude; we see from some of the Convocation crystals, e.g. Emmerololth, Mitron, Lahabrea and Elidibus, how resolved they were to save their star.

Venat gave her people no opportunity to succeed and actively sabotaged them - which makes her no different to Thordan or Ilberd who are both called out for doing as much even in the name of a greater goal.