Eorzea has much Light? How?

[Fenral]

On the topic of Hydaelyn and Zodiark once being a single being, though... I've had that as a theory for a while, now, but has it actually been confirmed? When did that happen?

Encyclopaedia Eorzea Vol. 1, page 14:

'From within the Mothercrystal, the will of Light--Hydaelyn--cast judgement that Darkness should be physically excised from the planet, and cast the will of Darkness--Zodiark--into space, where it became the moon[...]

'[Which is why the Ascians want to] wake the exiled Zodiark and restore Him to his rightful place back within Hydaelyn.

So, they both were part of an original Mothercrystal, and while we now equate Hydaelyn and Mothercrystal, it's only because she (technically speaking) usurped full control from Zodiark, who was split off into a moon. This entire section is sourced from the Scions themselves, though, so, ahem, "new revelations" could always change this interpretation down the line.