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

[Iscah]

Zodiark is the original will of the planet. Hydaelyn basically booted him from his chair and broke him into 14 pieces, 1 for every shard and then the Source, and imprisoned him in the Moon. She then sat down on his chair atop the lifestream and became the new will of the plant.

You may have misunderstood the order that things happened. Hydaelyn didn't break Zodiark into "one piece for each shard" because there were no shards at the time.

By our current understanding at least, it was the act of sundering Zodiark that (intentionally or not) shattered the world in the first place.