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

[Cleretic]

This is incorrect, the current version of the theory supposed that the tempering protects against the effects of those Primals for whom the Echo wouldn't suffice (read: Zodiark), not that the Echo offers no protection against tempering which it demonstrably does.

The argument still holds. Ifrit and his followers in that scene both provide the only mention in the entire game that being tempered protects from other tempering, and the most clear and steadfast evidence that the Warrior of Light is not tempered (namely, that Ifrit can sense tempering and says we're not; first-hand evidence, how about that).

How does this theory hold the former as a gospel truth, while throwing out the latter like yesterday's garbage? Surely, both should be held to an equal standard; either both are true and should be considered as such, or both should be thrown out as invalid evidence. It can't go both ways.

I’m also confused about the rambling on people calling Hydaelyn a goddess, people praying to her, or her calling us “child”. None of that necessarily = tempering.

If anyone is tempered by her, it would be Minfilia when she goes to her. But pre-HW Minfilia, the entire Path of Twelve, the Circle of Knowing, the entire country of Sharlayan, and the Eorzean leaders, all call her a Goddess or at least recognize her as the Mothercrystal, and only those in the Path have the Echo. None of them fully understood what she was prior to Emet-Selch's explanation and as it is in all societies in the face of something powerful to them, many called her a goddess.

Also, on this subject, it's worth noting that Hydaelyn worship to that degree is actually extremely minor in the game world, and not really held by any external commonality. Not everyone with the Echo worships her; Fordola, Misija, Arenvald and Mikoto never show any indication of believing in her. Not even everyone with the Blessing worships her; Ysayle recognizes that she exists, but DEFINITELY doesn't worship her.

Hydaelyn worship's just a small religion. If you distrust Hydaelyn worship, do you distrust every single religion in the entire game world? Because a huge theme of the game is that doing that is an awful thing that leads to horrible events.