Your first paragraph i agree it s exactly how it can go whit a mother/son relationship.

The reason of Sephiroth superiority isnt only cause Jenova isnt at full power.
But also because Sephiroth is the best of both side.

Here is more reason here on reason why he chose his own decisions http://www.zelaron.com/forums/showth...796#post328796

The only thing Jenova did was providing the way.