Personally, I don't think Greiver was a real GF, I think s/he was a psyche out. Afterall, it does tell you that Greiver is the most powerful GF in Squall's mind. It doesn't give a history of Griever, it's origins, nor does it even tell you whether or not Greiver is even junctioned to Ultimecia. The fact that it doesn't attack like a normal GF, one big attack and then it goes away until the next battle, leads me to believe that it's NOT a GF, just a different manifestation of Ultimecia's power to throw Squall and company off.
So I think all the facts about Greiver as a GF are pretty null, to be honest. It doesn't behave like a GF, so why classify it as a GF? No reason to.
Also, I don't buy the idea that each Sorceress is simply an incarnation in an infinite loop. Like it's been said before, the game itself says no one really knows how many sorceresses there are. Besides, if all the sorceresses ever WERE simply an incarnation of the same being, then those soceresses would have the exact same personalities, which we can see is not true. Adel was an evil being and abused her power. Edea was a good woman who thought she could best Ultimecia's evil, but failed. Rinoa just happened to be there and happened to have the right potential for the transfer of power and possession. Honestly, I believe it was unhappy coincidence that found Rinoa in her position. Yes, she had all the right potential to be a sorceress, but that isn't to say there weren't others in the world who did as well. Rinoa was the only one with such potential close enough to Ultimecia (while she was in possession of Edea), so it only follows that the possessor (Ulty) would immediately take over the closest avaliable host body (Rinoa).
Additionally, it's fallicious to assume that Rinoa inherited any powers from Ultimecia or Edea. Ultimecia never gave up her powers to a decendant, she just used them to control the others.
So while Edea had the potential, she was never a "real" sorceress, she just had the benefits of Ultimecia's powers. Rinoa, however, is a bonafide sorceress, in that she inherits her powers from Adel, upon Adel's death. (That's how I saw it anyways). So there are no splits in the chain that would suggest weird things were happening because of time compression or Ulty moving backwards in time to see herself. The line went as follows; unknown sorceress before Adel -> Adel -> Rinoa -> whomever her descendant is.
But going back to a point I was trying to make earlier, Rinoa is GOOD. She was possessed, yes, but in the end she uses her powers for GOOD. She uses them to benefit all of humanity. Rinoa was unselfish throughout the game. Even though she didn't want to, she was prepared to go to the sorceress memorial just to protect the people she cared for. Call me crazy, but I don't think people's primary instincts change just because their loved one dies. Rinoa realized that Squall could be taken from her at any moment, and still thought they should press onward.
Finally, I'd like to point out it's also fallicious to assume that sorceresses have any especial longevity. We know that Edea is in her forties, at least, but we are never told exactly how old Adel is. Regardless of what power they are capable of, sorceresses are NOT immortal, they are still very human. If they were capable of any sort of immortality, there would be no need to pass on their powers, or even feel threatened by other humans, but they ARE, which is why they attempt to enslave them. Point being, as far as we know, sorceresses live for a normal amount of time, just like other human beings, because they are able to DIE, just like other human beings (the proof here, obviously, is both Adel and Ultimecia. Both died.) So, while it is always POSSIBLE that a sorceress has a longer lifespan, I would say it's highly improbable, and a rather weak argument to use in favor of R-U.
Adjacent to that point, I assumed all along Ultimecia's mission to compress time was to become the supreme power of the universe and be God. So how did she know about Ellone; who knows. Maybe Ellone goes on to later expose her talent and goes down in history. Maybe Ultimecia is technologically able to see back in time (as I believe the game suggests she is).
But, like it's been said before, it's only theory. It would be fun to involve some philosopical proofs in the matter, but I am lazy and do not want too.
EDIT: I completely agree with Skyblade in his/her last bit, especially with the Arwen/Aragorn parallel. Even if Rinoa WERE capable of outliving Squall, would she? Even if she WERE capable of owning the whole world, WOULD SHE? I think the answer is no, Rinoa is not innately selfish, she puts others before herself and she's not easily swayed by what goes on outside her. She follows her heart, and I don't think she is mentally capable of trying to destroy the world, even if she did go nuts.