Future Esthar, the problem with your speculation is that it's just that: unfounded speculation.
On the same basis, I could argue that Irvine is in fact Ultimecia. It's simple enough really. He was in fact a woman trapped in a mans body(his obsession with women isn't 'cause he's attracted to them), something which slowly drove him insane. He then kills Rinoa and takes her powers, which he then uses to transform himself into Ultimecia. He then goes completely bonkers and decides to compress time so that he can be a female sexgoddess for all eternity.
The theory I just presented has as much factual basis as your own theory.
So really, unless you actually back your theory up and refute the arguments Squall of SeeD used, your theory has as much validity as Irvine=Ultimecia.
By the way, Squall of SeeD disproved your theory with the fact that Ultimecia CAN'T be possessing Edea after disc 2, because Ultimecia can only possess sorceresses, and Edea lost all her powers at the end of disc 2. You conveniently ignored that bit though...