-We learn that summoning being at the level of Zodiarc does not mean tempering similar to eorzean primals, that was simply an addition to the summoning spell, for beings such as zordiarc you still feel a "tug" which can be described as a form of tempering but nothing like what we expect from the average tempered individual, Emet Selch himself was evidence of that after all since shb.
-Zodiarc was not an evil cunning manipulative entity, like all primals he was a tool serving a function, most should have known that since Shb. Meaning as long as he existed, he would have kept the celestial currents going protecting eithyris from the song of oblivion giving the convocation enough time to figure out what to do.
-The souls that were sacrificed to summon Zodiarc where still around, within Zodiarc, so we know absolutely know Zodiarc could bring them back if additional sacrifices were given. And then the WoL goes in and kills them all by killing Zodiarc
(The good guy btw)
-We know that the concept of dynamis already existed even though not as widely known, and they also knew that the events of the final days first appeared in areas where the celestial currents were weakest, given time it only makes sense it would have been figured out.