The act of tempering, as well as the hunger for aether to continue to grow are flaws in the summoning process added at some point by the Ascians.

We know now that Hydalin does not temper, she shields by locking down the aether of a person, her travel warding spell of her own design.

Did Zodiarc temper the others? Was it just the one who was his core who was driven by the need, those who particapated in the summoning, or everybody who became exposed?

When did the summoning gain the flaw as clearly the ifrit made to stop the volcano was harmless, it was not before the depletion of ancient life...

Earliest records we have though are all post sundering, and with all the disasters of rejoining some of them are indeed pretty sketchy.