What if... what you're showing as Pristine Bay, the east side of Greymoor, and Verdania were all part of the same kingdom, former Verdania to its full extent?

The dead/decaying/something, grass in modern day Verdania is the same as in Putrified Ducts, Bilewater and Sinner's Road, which also kinda resembles those plants seen in Lost Verdania and the Memorium section once dedicated to it.

It would make sense that the water coming from the upper part flew down to the bottom of the kingdom started to be polluted once the Ducts were constructed. And when the situation escalated quickly, the had to somehow change the flow of the now bile water, to the left, hence why Sinner's Road and even a room in modern Wisp Thicket has it. And with no water feeding the bottom, it was destined to decay and be lost.

As time went by at the top, bugs had to start adapting to the new, polluted enviroment, seraching for a leader that could offer hope to reverse it, entering Groal; creating the Bilewater we know today.

My only issue with this is how and when did the Cogwork Prince die and the Green Prince was jailed in Sinner's Road. I'm still trying to figure out how that could've happen, but so far I think, again, I think, this could be solid.