It is possible that Rinoa is Ultimecia but not propable. If Rinoa was Ultimecia and Squall died (Somehow...) and she went crazy and got Ultimecia's powers, her personality could have changed to match Squall's from thinking about him so much.
Against that, if she was thinking about him so much, why would she try to kill him in the future of TC? He looked just like she would have remembered him, fur lined short jacket and all. Wouldn't seeing him again give her some semblance of sanity back?

Another thing, Ultimecia summoned Griever in the final battle and he turned into some ugly purple and red lion, Rinoa, evil or not, would not want to desecrate Squall's memory by turning his favorite piece of jewelry into a weapon, especially considering that Squall gave her a ring with Griever's likeness on it.

Ultimecia looks like Rinoa, but since they never do a CG movie with her, she also looks like every other pale skinned, long haired, busty brunette in the game, given the PS1's unfortunate lack of picture quality.

Adel could have been killed without giving up her powers. Think The Covenant. It's the body that breaks down, not the powers. If they would have shot the seal into the sun, the seal would have held on long enough to get very close and by the time it gives way, it's too late for Adel. Shell, Protect, Reflect. None of them would have mattered.

And Cid is not Squall. Cid was a grown man and married to Edea by the time Ultimecia gave her powers to Matron. Squall was like 7 when that happened and still running around in his Charlie Brown T-Shirt.