The whole notion of being able to exorcise the sorceress power (as Doctor Odine himself says in Disc3) disproves the theory totally, even the longevity debate. If Rinoa realised that she would someday become Ultimecia or not, I'm sure she would eventually exorcise her sorceress powers. And don't say it cannot be done because Edea went to Esthar so Odine could 'remove' her powers, and he said it could be done. How, who knows, as they never elaborate. But the point is it CAN be done, according to the game. There is never anything that says it cannot be done. I think the R=U theory is attractive as it adds a whole new dimension to the villain, but I think it is far more plausible that Ultimecia is in fact a descendant of Squall and Rinoa and therefore possesses the ring, Griever. On the other hand, that may not even be the case, she could have simply extracted the unknown GF from Squall's ring in battle, whether he is in the party or not. The game probably just takes the story from the assumption that he or Rinoa would be in the battle.
Rinoa is not Ultimecia. Period.