Woah. I'm an English major, basically a trained foot soldier in dissecting textual and contexual evidence, and even I think you are reading WAY too much into things here.
When I read your first post about Ulty possibly being Julia, I didn't believe you, but yet I thought it was an interesting idea and thought that the evidence you suggested was thought-provoking, although ultimately flawed - even IF Julia were alive, even IF she became Ulty, what is her MOTIVE?
Let's pretend we are a jury and Ulty is on trial for attempting to destory the world. But no one knows who Ulty really is. So the lawyers (in this case, you) start accusing people, like Rinoa, or Ellone, or Julia, or whatnot, with circumstanial evidence, but without irrefutable evidence and plausible motive, there is no way we could convict anyone.
Let's be logical:
What is Ulty trying to do?
Cause Time Compression.
What is that?
A world in which only she can exist.
Why the hell would she want to do that? Because she needs some quality alone time? Some summer reading to catch up on?
The only logical answer is the one TragicFaustus so brilliantly gave. That she can't survive otherwise.
Suddenly, the evil sorceress dooming the world to repeat this endless cycle makes sense. She wants to be freed from this cycle as well. She wants to exist. And the only way she can do that is to keep repeating the cycle until she wins.
Now, none of the people in the game's present - Rinoa, Julia, ect, have any problem existing the the world at its current state. In fact, were Ulty to succeed, they would all be destroyed (Including Julia if the game lied and she were still alive).
So in essence, because EVERYONE but Sorceress Ulty would be destroyed during Time Compression, the paradox twist works for ANY character, living or dead, past or future, NOT just Rinoa. Granted, the game ends if you are KO'd, but its enough that Ulty should not even TRY to kill herself. Therefore, because she attempts to compress time, an act which would kill all characters other than herself, Ulty CANNOT be anyone other than Ulty.
Square just screwed up having a villian who shows up so late in the story that we want to believe that there' more to her than that. There isn't.