The mural could be oriented either way in the room. The beam of light could match the top or the bottom of the mural, for all I know. Also, I don’t remember the journal saying the black moons match the beam of light, just that the moons need to move around.
The journal doesn't leave any room for interpretation and doesn't say anything about the beam. It says that the full moons should be aligned with stars (wich are always placed inside that top circle) and the darkness should stay below (dark moons go to the bottom circle).
I agree that they could stress a bit somewhere that Selune is all about light and casting shadows away even during the dark time (night).
