The questions I want EW to answer the most: WHY and HOW was the world sundered?

[Veloran]

Ok, I think I figured it out and if I'm wrong I'm going to unsub permanently and uninstall FFXIV from my laptop (I will post videos of me uninstalling FFXIV).

A few issues.
First, "Darkness" and "Zodiark" are used interchangeably just as "Light" and "Hydaelyn" often are. In the conversation with Word Minfilia she continually refers to the Ascians and Zodiark in the same context as "Darkness", and says that both Light and Darkness (IE herself and Zodiark) did once dwell as one. So if Hydaelyn/Zodiark are the "Light" there's no explanation for them being one with the Sound, nor an explanation for Venat saying they'll summon the "Light" if Zodiark already existed. And of course Zodiark was aspected astrally, not towards the Light. And the Sound "coveting power" hardly makes sense given it was simply reducing the entire planet to a lifeless husk.
It should further go without saying that the "Darkness" in that conversation being recontextualized as the Sound would be a drastic change to the point of retcon.

On the topic of tempering, I think it's very likely that WoL is indeed tempered. With what we know now, having one's aether attuned to that of a primal certainly does sound like the Blessing of Light, which connects WoL to Hydaelyn. And Hydaelyn's interactions with Minfilia and Ardbert, with the former being convinced of her godhood and the latter being forced to stop in his tracks with a wave of her hand, is a bit telling.