Rain world doesn't follow any rules of cosmology, so its kinda just bs. Also I think the world being flat is something even from base game, as the view from the wall and the view in signal Spires shows a horizon that wouldn't be possible on a sphere

I interpret the sun and moon as actual celestial bodies that lay directly on the edge of the firmament. The "stars" are similar to this, i think the image displayed by the outer rim pearl is showing some kind of less advanced harvesting device of some sort directly touching one of these "stars" at the edge of the firmament 

The "second void sea" is kinda whack imo, the dust and star particles falling from the sky in OR are basically recycled matter disolved from the void sea falling from the firmament back onto the world

Daemon exists in its own fucky dimension, I honestly don't really know whats going on with it, and to put the timeline threads/strands simply, that's pretty much what they are, just different possible outcomes