Ultimecia told Edea who she was and what she was trying to do. I can see Edea hiding it from Squall and Rinoa, in order to protect them, but why wouldn't Ultimecia tell Rinoa when she possessed her?
Rinoas was possessed in a different manner, that much is clear. When we see Edea possessed, she looks perfectly normal, not possessed. When Rinoa is possessed she turns into some freaky, weirdly transparent, glowing mindless creature. So even if Ultimecia couldn't hide her thoughts Rinoa would probably never know.

Furthermore, if Ultimecia WAS as deranged as the theory claims, she wouldn't even remember being Rinoa, let alone "becoming" Ultimecia.

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.
Girever IS a GF. That's made clear also in the japanese version. This is what it says to be precise:

Ultimecia: Your feelings, I shall summon the most powerful of things
[from them]! The more strongly you feel, that will be what shall torment you. Fufu.

After fighting Griever for a while:

Ultimecia: Fufu. Are your memories fading to nothing? This is not the
dreadfulness of the true GF. The GF's true ferocity, let me instruct you
[in it]! That power, show it to them! Griever!

---

Of course, from that there, and the scan info from Squall, it seems obvious that Ultimecia created Griever, the GF, right there and then based on Squalls emotions and thoughts. So you're right that in the end, Griever doesn't point towards the theory or against it.

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Ok, several other topics were mentioned, including sorceress lifespan, and the timeloop in conjunction with the cycling of powers. Well, I wrote an FAQ precisely on those things(and some other stuff), so yet again I'd refer you to it:

http://www.gamefaqs.com/console/psx/game/197343.html

It's the "Time/Ultimecia Plot FAQ" in the FAQ list(alpahbetically listed).
From there you can gather my view on things, and comment subsequently. I'd very much appreciate it if you could read it, and give feedback and comments.