Rinoa is a part of Ultimecia (not saying Ultimecia is 100% Rinoa though) because like what you said, Squall gave her the ring, Rinoa felt the power and eventually Ultimecia spawned Griever at the end. It seems to make more sense than Rinoa being a descendant of Ultimecia because I don't think that all descendants of Ultimecia would gain to have the same power as Rinoa (gaining Griever GF power), unless Rinoa gave her ring to all of the descendants but I don't think that did happen. If Ultimecia created Griever from someone's mind, but why only Griever, why not something else? Griever GF is probably the only one thing Ultimecia has created, but why?
So I tend to believe that Squall could be more of a part of Ultimecia because of Griever, you know when a GF is equipped, nobody else can't use it. Well squall used it for a long time, but maybe Ultimecia wore the ring as a GF and Griever eventually showed up, but where is Squall from the future (the ring is still alive)? He's probably dead, his ring has fallen to Ultimecia but how did that happen? All I guess is... Rinoa is probably the closest person to Squall to have possession of Griever ring, so thus, Rinoa is Ultimecia or a part of it or you're probably right, each descendant following to the future from the past gave Griever ring to next one, and so on, until ultimecia but idk.
If Gilgamesh went through dimensional travel, then Rinoa may have the chance to do that since she has some power of a sorceress, maybe it's not true but who knows. Adel held Rinoa, was that when Gilgamesh showed up? Would make sense if Rinoa was involved with Gilgamesh.