The tempered thing is a huge "so what?"
Revealing the true cause of the Final Days does not imply Zodiark would need to go. Quite the opposite. So it's not as though anyone is saying their god - who saved the world, restored it and continued to shield it - is the issue. Zodiark had to be maintained to form a shield around the world. Their entire dispute centered around those souls caught inside him; in order to keep him functional in his role, it is plausible he'd require an exchange of aether to release them. Also, we are looking at Emet 12k years later, and while he does say they're tempered, he frames this in particular around Zodiark's aspect of darkness and the Ascians acting as transmission belts of that. This is a known aspect of tempering, alignment to the Primal's aether, but it is only one part of it. As a primal lacking a presiding will to compel them to do anything, and without the flawed rites used in the beast tribe summonings, I am wholly unpersuaded that said tempering would pose an issue in their discussions or to them believing Venat's story, particularly when Elidibus himself emerged from Zodiark to mediate a dispute. Considering what Zodiark is (an entity formed of the collective will of his people to save their star), I am not surprised that Emet exhibits reverence to him... and even so he was willing to give the WoL a chance and deviate from his original plan... 12k years later.
Regarding Emet's concession on their methods, there is not much reason to take this to refer to ancient society as such, when we could simply go with the much more plausible explanation of it referring to the Rejoining plan, which is not really what Veloran is arguing about - which is to say the scenario where the ancients were given that information before the Sundering. And I have to say, unless she were to spill the beans on her reasoning and ditch the platitudes, I see very little reason as to why they ought to heed her and forestall releasing their people from Zodiark.