The matter of Rinoa being Ultimecia is really baseless. It requires the assumption that Witches are immortal, something that the game doesn't suggest in the first place. For any points that are used to argue in favor of the concept, there's an in-game indication or a far more simple explanation that can be reasoned logically that has nothing to do with the concept.

For example, people often cite Ultimecia's castle being near/facing the flower field where Squall and Rinoa promised to meet as being evidence of this. However, there are fresh bodies of dead White SeeDs littering the beach at the Orphanage, which would suggest that Ultimecia's castle was there for her to take out the last SeeDs to settle her grudge with them before casting Time Compression.

As for Griever being Squall, that's directly contradicted by the game, which makes it clear that Ultimecia created Griever on the spot during that fight from the feelings of her opponents. Griever was, in Squall's mind, the most powerful of beings.