There are missionaries for R=U now? What’s next, St. Heartilly’s First United?
It is entirely conjecture. The word conjecture means conclusions based on incomplete evidence. The game offers nothing conclusive about time, roles, even the laws of physics, as characters apparently get shot without suffering death. There are a great deal of assumptions, including lifespan and future events being what they are. There’s also the issue of GF’s. We have no idea how they erase memories, the long-term effects, or the frequency of use, if memories are just suppressed or ever become erased permanently. There’s no reason to assume that Rinoa’s facial features were somehow ‘altered’ by any means. No idea how the nature of sorceresses death works, in regard to passing on their powers. And no idea about the nature of Griever. All the points trying to prove the R=U theory only make sense while already assuming the theory is true. That’s called circular reasoning and is never acceptable in an argument as locical.
If you want a good example of a solid theory with evidence that’s not conjecture, look at Adel’s tomb affecting radio waves, or Laguna being Squall’s dad. It’s never explicitly stated but statements near the end of the game, and the moombas in D-District show that it’s the case.
The simplest explanation is often the best one. And since there is nothing that explicitly connects Rinoa and Ultimecia in a simple manner, we must conclude that there is no evidence that Rinoa and Ultimecia are the same person. The best evidence that they are not, is that Square never put anything in the game about it. If Square thought that Rinoa=Ultimecia would be a good plot point, they would have shown something about it. The only evidence these people have to offer are combining game physics that were used as plot points in unrelated parts of the game (Ex. memory loss for the orphanage). And that’s not conjecture.
An excellent diatribe on this topic can be found at GameFAQs “Time/Ultimecia Plot FAQ”, which, in much more detail than you’d ever want, disproves the R=U theory.