The universe thing is a bit weird, the logic leap of MC that thought that the black star was Earth makes no sense to me, I would think is the demon world launching a different invesaion or straight up a magical meteorite.
The reversed constellations too, I would just think that it means the planet MC is on is on the opposite side of the stars in the unvierse from earth perspective.
If is a mirror world in another dimension Earth would not even enter my calculations, this is the author foreshadowing stuff for us readers as there is no connection to the 2 otherway, but leaving enough space to still drop it later as MC being wrong.
Like Bug, clearly was intended for something bigger but got rewritten into a supporter without drama.