I think one of the biggest differences between Emet-Selch and Venat in this context is what they consider "their people." It's also tied to the wider conversation about how Ancients viewed death. Emet-Selch says that post-sundered people's aether is so weak he doesn't consider them fully alive, and believes they'd stand no chance against the Final Days (until he finally decides to give them a chance at the end of 5.0). But for Venat, she views sundering as a sort of limiter she's placing on her people that in turn gives them the potential to find hope amidst despair and face Endsinger. So Venat still very much considers sundered life "her people" -- they are souls of the Ancients in new form. Because of the lifestream and the reincarnation of souls, sundering keeps her people alive, as opposed to the alternative proposed of sacrificing more life to Zodiark (which keeps their souls trapped, and might not lead them any closer to a solution). Even the rejoinings, while unbelievably tragic, still result in the joining of souls -- death is only transient, so long as she can preserve the cycle of rebirth (and not allow the Ascians to accomplish their ultimate goals).
But the other point here is... even having accepted that she wouldn't be able to stop suffering from occurring didn't mean she could just let it happen unchallenged. Her power was weaker than Zodiark to begin with and more limited after the sundering, but she had to assume that all those bad things happened despite her best, most earnest efforts to stop it. If she had just accepted the inevitability of what we told her and done nothing to intervene in all the years (acting purely on faith that the future was inevitable and would work itself out), that could also have broken the time loop and made things even worse for us (or resulted in everyone just killing themselves off or otherwise not surviving to be able to face Meteion). So given that she didn't know exactly what she did in all those intervening years, she had to always strive fervently to nurture hope despite having birthed a world of suffering.
Personally I don't think there's a discrepancy in pre-Elpis and post-Elpis Venat, provided you can accept that she considered this to be the "nothing is impossible" best way to save her world and her people -- that she earnestly believed the only way forward was to accept the "I acknowledge all must suffer to have the best chance to save the future" checkbox. And yeah, as Yoshida said, that's one heck of an agreement for her to unilaterally sign on behalf of the entire planet, but in the end she proved that it was at least "a" path forward. (I would still also point out again that this was a path that Emet-Selch himself had opened for her to pursue as well through his actions in Ktisis Hyperboreia; he knew that this is one of the possible futures that could result, even though he didn't want to believe it.)
Perhaps you could also say that, if you follow the sort of multiverse of time-travel theory, we live in the future where she did pick this path, and so that makes her a hero from our point of view (because otherwise we wouldn't be alive). But perhaps there are a whole lot of parallel time dimensions where she made different choices, some of which worked out in a different way, some of which ended with the Ancients calling for Ra-la to kill them. We can't ever know those other paths because, from our point of view, those other paths never existed. Who is to say that we live in the "perfect future," but it is a future where we exist, faced the Final Days, and lived on to tell the tale -- so, from our (the WoL and friends') point of view, it's the good end.