Apart from Ultimania, I have yet to see anything which convinces me R=U is even close to impossible or even very unlikely. You can infer how a random individual could have become Ultimecia, but that seems a lot less interesting, and far less involved in the 'fate' theme running through the story.
It certainly isn't "impossible", because anything is strictly speaking possible. The only problem is that it isn't actually implied by the game to be true to begin with, which is why it IS unlikely. You agreed yourself that the hints for the theory come from backwards reasoning, which is enough in itself to demonstrate why the theory is unlikely to be true. If it isn't suggested by the game, it cannot be considered a valid theory. It's that simple.
As for the theme of fate, the game itself readily implies a background for Ultimecia which links strongly into the theme of fate. That is, that Ultimecia was persecuted before doing anything because her fate was known to the public (she died in the past, remember, so she would have been as known to the FF8 world as say Hitler is to us), and in an attempt not only to get revenge on the people, but also to avoid her own supposed fate of dying at Squalls hand, she tries to inititate time compression, thus setting in motion the events that lead to her death. Perfectly fits in with fate, is implied by the game AND the Ultimania and doesn't require any baseless assumptions about Ultimecia really being a main character. It just doesn't make sense to try looking for technicalities that can allow R=U to work when it isn't really backed up by the game and the game itself offers a perfectly reasonable background to begin with!
The expanded theory described above can be read in the FAQ referred to.