If the Echo prevents tempering, how were the Ascians tempered by Zodiark? It’s been heavily hinted that the Echo is either an innately Ascian power, or a power Ascians possess. If it prevented tempering, they too would be immune.

Whether we are tempered by Hydaelyn or not is almost irrelevant, though (fwiw, I think not). Her commandments of Hear, Feel, Think are essentially the basis of free will. If we are slaves to her will, and her will is that we have free will, it’s distinction without difference.

I think it’s clear at this point that Hydaelyn represents liberty, and Zodiark represents order. That’s the essence of the split between the summoning factions. Emet-Selch’s obsession with “perfection” is proof enough that order is their overriding concern, and we saw this in the glimpses we got of Aumarotian society. It’s even clearer if you’re familiar with the work Aumarot is derived from - Utopia - which parodizes the application of philosophical ideals to practical governance by showing the absurd extremes to which a society must go to maintain order. Many of these are featured in the quests of fake-Aumarot, such as everyone wearing the same clothes because free expression is egotism and a danger to society. This is what “the Sound” which precipitated the crisis which befell the Ascians represents: discord. Perfection is a fragile thing.

The Hydaelyn faction, meanwhile, objected to the forced sacrifice of life to Zodiark. They believed the future belonged to those who lived, and that it was wrong to take that future from them against their will. It’s significant that this was the point of their objection - after two sacrifices had already been made to Zodiark by the consent of the sacrificed.

It is likely that Hydaelyn was summoned with the express purpose of defending against the dominating will of Zodiark - hence her ability to protect us from tempering, aka the Blessing of Light. But in doing this, the system by which Zodiark restored order was threatened: his dominating will was the mechanism by which the volatility of Creation magic was kept in check. This made their coexistence untenable, and so they were destined to conflict. From there, it’s easy to see how we arrived at the Sundering.

Zodiark was the will of the star made manifest, and outright destroying him would doubtless have consequences, if it were even possible. At the very least, his destruction could return the world to how it was before he restored it. The summoners of Hydaelyn did not want this, obviously, because they went along with his summoning in the first place. So the question became how to ensure the agency of the living, while also neutralizing the threat of unchecked Creation magic.

This might have all been obvious before Hydaelyn’s summoning, hence her being given the power to Sunder in the first place, perhaps as a “weapon of last resort”. This would also make sense given that they sacrificed themselves to summon her. It’s also possible that in doing so, Hydaelyn’s own summoning was undone - whereas Zodiark was apparently physically banished and split, Hydaelyn seems to exist in an unsummoned state, regulating the aetheric balance. This might explain why Rejoinings and the summoning of Primals weaken her.