A reasonable suggestion, probably MORE reasonable than R=U and the same assumption I had initially, she hears about this Squall and Co. who supposedly kill her at some point (with the Time Compression going on the actual event could have happened ANYTIME once time straightens back out)and heads back to stop it, maybe compressing time just so she can find the point where she was killed. Regardless, it's just an opinion, equally valid to going back to happy times which also seems like a pretty decent reason to...kill time, I'd do it.
Which IS alluded to by that quote about Squall being her knight forever 'Even if you become the enemy of the world' or whatever which would be some interesting foreshadowing if the theory were true.
Perhaps she chose Siefer instead by accident because of the parallels between him and Squall? And the fact that she had a thing for him at one point too. Her memory is clearly a bit off
I mean, the scar could have brought something back or just the relationship, far-fetched but whatever, so are all of these theories.
I'm pretty sure it's just a regular lion on the ring and necklace that Squall REFERS to as Griever, and the Griever GF is just a twisted version of that. Whether that monstrous form comes from Squall's mind or it has been twisted that way by hanging out with Rinoa as she transformed into the equally twisted Ultimecia is debatable
I find it difficult to choose a side on this, I'm usually against the people who grasp at straws to come up with this sort of thing but there is so much evidence on both sides (even if it is all conjecture) that to me at least it has come to the point of more or less believing either way because of choice instead of a solid consensus in my mind.
Either way, while I don't really believe it fully, it is definitely fun to discuss because it certainly would add some depth to a villain which to me on my first playthough was basically just another 'Ultimate Evil' sort of thing, despite some other theories (and they are just theories, plausible as they may be) fleshing her out a bit more.
Also, slightly off topic but does someone have a link to a full translation of the FFVIII Ultimania? Because I've only ever found one for IX and the IX Ultimania had MASSES of info not even briefly mentioned in the game, I'd like to see something like that for VIII.