Well, I' m a noobie in FFH and so I will NOT argument with lore, but with what I know in real life.
I have a pretty good knowledge on mythology and science so I have a *proposal* of *interpretation* that can put everything together.
I mean that it is not what actually say the lore, but a new perspective on it.
What IF those FFH myths are, in fact, only myths?
Just like our ancient ones... they are even pretty similar to them: world considered flat, located between heaven and hell, and all other things common to ancient myths.
Btw, this in Erebus inhabitants' mind -> ancient people with no modern scientific knowledge.
So, Erebus *could* actually be a planet (like Earth or Mars, but btw an Earth-like one) in space, but for the ancient civilizations that inhabits it, who doesn't have our modern knowledge, it is flat and unique.
Just as we did believe millennia ago....
The portals which link various worlds which the goddess is said to visit, "even beyond those created by the gods of Erebus" can actually be other planets orbiting other stars, etc.
And maybe, all of this Universe is just the whole Creation made by The One of the Erebus myths... Who knows.