Look, Serapy:
Viator's theory is no good because it rests on the assumption that what is really a tiny plot hole, is vital to the entire plot. That's it. The arguments you are brining up back and forth are really not necessary, because they rest on the foundation they do.
However, the R=U theory is not plausible either, because it reads far too much into the game, and simply isn't at all plausible.
Ultimecia's main motives were, rather, a mixture of a desire to become the new God (time compression would allow her to absorb all of time and space including all the sorceress powers, making her God) and a desire to conquer her recorded fate of dying at Squall's hand (remember her death happened in her past, so it would have been recorded). That's it.
A more complete analysis of the faults of R=U, and of the just mentioned theory on Ultimecia, refer to the FAQ probably mentioned already in this topic.