What causes Nirn's gravity and seasons? From what I understand, Mundus is a realm surrounded by Oblivion which in turn is surrounded by Atherius. Magnus is a hole in Oblivion to Atherius, and such Nirn wouldn't orbit it, assuming I understand correctly. So what causes the seasons and orbit? (apart from the magic of a dead god?(Lorkhan))

Arkay controls the seasons.

The gods are (Willing or not) the equivalent to the laws of physics. Things from time to the form living creatures take are the result of these beings.