Actually, the choice of Rinoa has a clear canon reason--the capacity for Sorceress power is basically determined by innate magical potential. If we dip into stats, then that's 'magic' stat. Rinoa had the most potential--Rinoa got the powers, unless one of the others were specifically chosen.

The problem is that you haven't /proven/ anything, though. What you're using isn't proof; it's bad reasoning. it's saying 'look here's a flaw that I just made up! Now believe me!'