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

[Cilia]

Amaurot's flaw was that it was complacent.

We know from Emet-Selch's testimony (backed up with evidence) that Amaurot was a virtual paradise: a city-state (or just state outright) with no strife, virtual equity, and everyone living in peace and contentment. This is exactly what brought about its fall long before the Final Days, let alone the Sundering.

The Sound affected other areas of the planet before Amaurot... but they seemed mostly unconcerned about it except in the academic sense, performing some experiments and trying to come up with a plan, but nothing concrete (even though phantom Amaurot is taken from the day just before). The Amaurotine way of life offering nothing but comfort meant that when the Final Days actually reached them and stressed them beyond their limit, they had absolutely no idea what to do - despite their supposed perfection - and resorted to mass sacrifice to create an artificial deity who fixed the symptoms of the problem. Then faced with a changed world, the Amaurotines (or at least the Conclave, which enjoyed overwhelming popular support on the issue anyway) decided the best thing to do would be to return to the former status quo at great cost to the rest of the planet (whether that included sentient life or not is... kind of irrelevant to my point), which aside from trying to control the future of the planet and everyone on it sets a very dangerous precedent; more importantly it shows Amaurot's unwillingness or inability to accept change, a desire to exist and be in control forever, and a conceited belief that their own lives far outweigh all others (sentiments still shown by the Ascians).

It's not for just a(nother) XII reference the leader of the faction opposing the idea was named Venat, I don't think.

In other words... Amaurot fell the moment it embraced the impossible concept of perfection, unwilling or unable to adapt to change due to the peace and contentment enjoyed by its citizens. For all its glory and majesty, Amaurot was ultimately a dead end for civilization. The Final Days and Sundering just pushed it over the edge, whatever the cause of the former may have been.

That's not to say the Final Days or Sundering were necessarily "their own fault," that they "deserved [either]," that there was seedy stuff going on behind the scenes we just aren't aware of yet because the phantom Amaurot we visited was entirely tinted by Emet-Selch's nostalgia, or even that "good" or "evil" factor into this in any way. Just that Amaurot was not perfect - and in fact the unending peace and contentment enjoyed by its citizens is precisely why it wasn't.

P.S. We don't have enough information on every in-game civilization (past or present) to identify its shortcomings, but they're all bound to have some provided enough information. We know less about Skalla than we do about Amaurot, though, so...