In the end, it ends up just being a question of what the sacrifice should be and what should be the result. They basically all believed that there is no concrete sacrifice not worth making for the sake of Etheirys's future, but disagreed about what that future should look like.
The key fork in the road here happened when the Convocation decided they were ready to seed and sacrifice all non-Ancient life on the star to bring back their own past sacrifices. If they did that, how does that get them any closer to beating Meteion? Doesn't it entirely prove the reason for Hermes's own despair with Ancient society to begin with (at how little they valued the lives of those they considered inferior to themselves)? Doesn't it strongly support the argument their that their own decadence was leading to their inevitable downfall regardless? So it's at that point (to counter the third sacrifice-to-Zodiark plan) she decides that pursuing the sundering path that led to the WoL was the more hopeful option. She was told about the rejoined shards she wouldn't be able to save, but better that (and to have their souls rejoined) than to kill all other life on the planet just to appease what is essentially the Ancients' inability to accept the pain of loss.
I'm not so sure that all this is because she became enamored with the WoL to exactly the degree stated, but at the very least she saw a path with potential there, despite knowing the cost it'd take to reach it. So the decision she had to make was to weigh the cost of that path (and all the sacrifices involved) with the cost and likely result of the path the main faction was pursuing. They were all gambling with lives at an unbelievable scale. With respect to the criticisms offered, perhaps it could have helped to spend more time covering that interim period, all the alternate things that were tried, and exactly what led to Venat realizing she had no choice but to go this way. (Clearly it wasn't her first choice because she let the Convocation do the first two rounds of sacrifices to the Zodiark; she wanted them first and foremost to pass Hermes's trial on their own. But the third round was the bridge too far.)