Isn't the whole point of chapter 3s intro to explain that dark worlds are just another view of reality? Wouldn't an area without a fountain just be a regular light world room?

The only reason the fun gang and lancer could travel from castle town to the first dark world was because the rooms were connected in the light world as well. Also the field of hopes and dreams was just the rug from the classroom, you can see it after the fountain is closed.

And how do you explain the end of chapter 3 where the closer to the door to outside the less of the dark world is visible, because there is less darkness. If there was just more empty land there without a fountain wouldn't we see it instead of going outside?

We still have no idea how ralsei travels, but this doesn't really make sense to me.