I'm not some huge lore buff but all this doesn't make a lot of sense. There is night and day and season, so there must be a sun and the planet must rotate. There are stars in the sky.
Several of the month names have "sun" in them (Sun's Dawn, Sun's Height, Sun's Dusk) so there must be a Sun. I don't know what Magnus is and I looked it up and all I got was that it's the name of some god.
Instead of devising all of these "theories" it is easy to just see Nirn for what it is. A spherical planet with two moons in an orbit around a star, neighboring many other stars.
These aren't theories. These are beliefs as written in the lore. Sure they can be wrong, but we don't want them to be, and it would make all the effort that went into them pretty worthless. The whole http://www.imperial-library.info/obscure_text/cosmology.shtml document is a delightfully playful Borgesian spin on a very mundane subject. That, in case you were wondering, is cool.
Magnus is a sun and a hole in the fabric of sky leading to Aetherius. Simple, eh?
When it's night time, looking at the sky is staring at the sea of oblivion, and stars are the pinpricks at the edge of that sea. When it's day time, you're staring at Magnus's technicoloured dreamcoat, and the realm of aetherius. Seasons aren't explained in the text, but could simply be the influence of elemental forces, caused by anything from Y'ffre to the stars to variations of magick.