Let's play a game: Save the Ancients!

[Cleretic]

For the second option, we could convince Venat to share the upcoming Final Days with the Convocation and also gave them a better solution. Instead of summoning Zodiark to shield the planet with Aether to block out Endsinger's song (which would only be a temporary solution and lead up to the sundering as in our timeline), summon a primal of Dynamis filled with positive emotions to do battle with Endsinger. Venat put a tracker so they could easily find her nest.

NOPE

I'm sorry, but I need to bludgeon you with why this is a bad idea, and if this thread's going to continue it needs to be on the first page. There is a reason Venat did not do this, and if we're doing textual intervention we need to understand that reason.

Essentially, Venat correctly recognized that telling the Convocation only creates problems. The easiest way to understand this is that the main people to worry about are exactly the two people we see respond to the information in the first place: Hermes, and Emet. They can't tell Hermes, because if he learns that the End of Days was Meteion's doing, he becomes too depressed to help, and possibly goes full traitor; both of those are bad, because he's the only dynamis expert in a position to actually enact anything resembling a Zodiark. (He might actually be the only dynamis expert around at all, for all we know.) And we can't tell Emet, because he's a stickler for rules and process; if he learns, he will do the same thing he did in Elpis, which if you remember, caused a lot of those problems. This is one of the two reasons she's selective about who she tells the truth to; if word gets through to the Convocation at large, everything's lost.

Honestly, the rest of this hypothetical is purely baseless what-if that I don't think can be worked with in any way; not only do I not think the Ancients can actually reach and traverse Ultima Thule, I'm not even sure a 'primal made of dynamis' is possible, that might be akin to asking for a sword made of helium. And personally, I also don't think this solves things; I think that's essentially a Zodiark of a different shape, you can't make something like that without some major cost, and after that I think an equivalent of sacrifices and Hydaelyn of some form are inevitable.

That said, you have gotten me thinking, and I think that if you really, really, REALLY wanted to tell the Convocation, there is one angle that doesn't hit this failure point: Lahabrea. The only Convocation member who can keep a secret, and also a man who's willing to do some work in the shadows for what he sees as the greater good. Now, I don't know if his assistance actually improves things, and in fact, I could argue that turning him actually might make things worse: he's also a big part of making Zodiark, so it might just result in a worse Zodiark from him not helping as much; I think this might just result in Bad Ending B rather than a Bad Ending A. But I do think that he's the only Convocation member that wouldn't immediately cause the failure points Venat was avoiding.