About Shadow Magic and its relation to the other elements: The game manual specifies that Shadow is the combination of the other three. Of course that could be just another translation but its supported by the game itself. The tech "Delta Storm" essentially a combination of Fire2, Water2, and Lightning2, does Shadow damage. I know because I relied heavily on it when Magus pulled up his "Shadow" shield and DarkBombed you.
The most powerful magic users seem to use Shadow, though whether or not it was the original form of magic is up to speculation.