Saying that Ultimecia is a different person isn't justified. You don't see much of her backstory in VIII, compared to the ones of the main characters. No, the Ultimania doesn't count.

Saying that R=U has no support or proof isn't justified, either. The plot of VIII isn't exactly a cat-walk in the ultimate maze. There are so many things that need to be answered in VIII.

This is obvious and simple. VIII has represented a lot of symbolic things even in plot areas. Understanding these things by yourself can affect the plot. Symbolic things (or symbolisms) AND R=U are actually the same thing. They don't explictly tell you anything about them, they just show you, and that's it. So, proving or disproving these things are plain silly and wrong. Applying occam's razor to VIII where symbolisms are mostly used compared to other FFs... are again plain silly and wrong.

If you're kind of that person who demands evidence or logic for everything, go play a different game. VIII isn't for you.