Genetics don't lie. Since I am not very familiar with hair color genetics, I will use eye color genetics instead.
Squall: bb (blue)
+Rinoa: BB (Very dark brown)
=Zell: Bb, Bb, Bb, Bb (brown)
As you can see, it is impossible for Zell to have blue eyes with Squall and Rinoa as his parents. This part of your theory is invalid.
