The way I see it, Magus is a sorceror, so of course he can use all types of magic. Shadow is of course his innate, is all.
The way I see it, the whole balance of Water, Fire, Lightning, and Shadow is symbolic. Fire and Water are opposites of a physical existence, and Lightning and Shadow can be interpretted as opposites of a spiritual existence (Heaven and Shadow.) And it continues the parallel, physical versus spiritual. This keeps folding out infinitely to all concepts of the universe. True Tao. Balance. But it's not so clearcut. You say Antipode, a Shadow spell, is made with Fire and Water? Ha. I present two reason. One, they're opposites, so they negate each other and create Shadow, Void, where Heaven would represent existence, or atleast the cause of it. Reason Two would be continuing the Tao analogy, as even opposites contain traces of their counterparts, as they can never be truly seperated. Nothing is fully good or evil. Right or Wrong. Black or White. The physical elements of Fire and Water cannot escape the spiritual Shadow.
Damn, my brain hurts.