They came across as well-intentioned extremists, and did not chastise the Convocation for being depraved, but for not entertaining their concerns - there may be a truth to this, in that the Convocation was uninterested in imposing further hardships on their people after what they had endured and had its own plans on how to restore the world to its rightful state, by bringing back their fallen. I am beginning to think that the reason behind why Venat's group did not endorse this course of action might not have (solely) stemmed out of sympathy for this "new life" to feature in the sacrifice, but out of a belief that because it lacked the same inherent potential for Creation magicks, it would not risk suffering the same fate as did the ancients, something the Convocation would regard as aberrant.
I am also partial to the theory I've seen that the 14th did not outright deny the Dissidents' request (even if they would frown upon a second Primal much as they would frown upon the first), but left the Convocation in the hopes of resolving the crisis before they implemented their plans, perhaps fearful of the risks of summoning Primals, especially on that scale. We know that the Final Days escalated rapidly, and it is possible that it took hold of Amaurot before they could return, resulting in Zodiark's summoning and the apparent end of the crisis. For whatever reason, Venat's group had different views, acknowledged the Convocation meant well but would not listen to them, and sought to thus force their solution on it by either "checking" Zodiark or ousting him entirely, and replacing him with what they deemed to be a superior solution. As yet, the Sundering does not necessarily seem to be intended, but Elidibus's moon epilogue in 5.0 does mention that the wiping of memories of the ancient world and of the truth of the sundered lifeforms' fragmented existences was intended by her summoners, so there is a reasonable possibility that so too was the Sundering, or that this enervation was at least intended for the ancients themselves.
A possibility worth entertaining is that this crisis originated in the Underworld, which based on Hades's short story, was a part of the "aetherial realm" the Ancients could not venture into and scarcely controlled but only glimpsed. The phoenix in that story, after all, involved an apparent anomaly in the behaviour of a soul, which by all rights should have returned to the Underworld. The nature of the Underworld makes it a big unknown and introduces the possibility of why the crisis was so poorly understood. We've seen a few references now to the Titans, the Chthonic (=relating to the Underworld) deities preceding the Olympians in Greek mythology, both in Eden, with names like Gaia (mother of the Titans), Erebos and Nyx (born from Chaos, supposedly Gaia's progenitor), and also in the symbolism in Therion's title, the Chthonic Riddle (the resolution of which marked the transition from the Titans to the Olympian deities.) Some interpretation of them is therefore a possibility. Regardless of the actual cause, perhaps then the 14th ventured into this realm to resolve the issue, and did not return, even if they succeeded in sealing away whatever caused the crisis (thus why it is currently an unknown.) With all this occurring during this whole mess resulting in Zodiark and Hydaelyn being summoned, it would then escalate to the point that it was too late, and the world sundered. This is consistent with Emet-Selch's apparent incredulity on the revelation of the character's Amaurotine identity. He knows you are the fragment of a sundered ancient soul, and a powerful one at that, but perhaps this specific one was deemed lost to the world. The 5.0 epilogue asks by what hand would this war between light and dark - ultimately, a war of two Primals - be resolved, and perhaps this is where the neutrality of the 14th becomes relevant.