I would consider your argument if I saw any evidence in the games that lent it credence. I don't have a concrete surety of exactly what's going on with the subtler aspects of the game, so I'm willing to change my mind if I see evidence that suggests other than what I currently think.

Some problems:

There's no reason for Sephiroth to change his speech pattern after his experience in the lifestream, especially not to that of Jenova, but he does--perhaps because it is not him controlling his body.

Notice all of the times in the game that you're dealing with Sephiroth, he gives a big speech or whatever, then disappears and suddenly you're fighting Jenova.

The Sephirova of the present does not refer to Jenova as "mother" any more.

There is every indication, from glimpses of the past to the player characters' reactions, that the Sephiroth of the past (the real Sephiroth) has a different perspective (and philosophy) than the Sephiroth of the present (actually Jenova). For instance Sephiroth thinks that Jenova is his mother and that she is a Cetra and that the Cetra were sacrificed for humanity's sake. Sephiroth does not want godhood, he wants revenge. Jenova, on the other hand, does not connect itself with the people of this planet at all and therefore feels that it should be the god of the planet (hence Jehovah + nova = Jenova).

I've got more, but I'd like to hear contrary evidence first so I know which direction to go with this.