Doubtlessly it's a thing of choice, that much should at least be clear in any argument. No proof can ever be made for a theory(by the definition of the word) and no proof against it has been found, though many have tried, that's for sure.
Only it's level of plausibility can be argued. For instance, the fact that...
SPOILERS
...Laguna is Squalls father is also a theory by definition, yet one backed up by so strong arguments that it seems ridiculous to not take it as fact. Similarly here, one should at least make a case for plausibility.
As for Ultimecias looks, they are of no importance. After all, consider Adel. Judging on her looks it's hard to say she's human, but she was just as everyone else a human when she first got her powers. So any change in looks between R and U have little to no significance.