I'll just drop my 2 cents on this topic, and walk away. Because someone somewhere will have an absolutely out of control shit fit about it, I'm sure.
Rinoa=Ulti is a dumbass idea for several reasons: one, they look, act and speak totally differently, and have totally different personalities as well. Like seriously, Ulti has one hell of a weird accent, and Rinoa just talks like a normal person. Not even the passage of time can explain that one away.
Second, there is no in game hint at any connections between the two. Except for greiver and they're both sorceresses. So why not Adel=Ulti or Edea=Ulti? Those two would make more sense, honestly.
Third, R=U makes no sense from a plot standpoint, as the plot is about the power of love helping a grumpy fuck get over himself and save the world with his girlfriend and buddies. If said girlfriend turns out to be the big evil, then what's the message supposed to be? The power of love will help you kill your beloved because reasons?
At best, the only spin that could work there is the team helping stop a tragic future from coming to pass, but if that were the idea, Square would have been beating us over the head with that idea from the end of disc 1 on. Trust me. And besides, the ending suggests that fate is inescapable, and shall always come to pass.
On the greiver topic. Squall and co are the fated children. Ulti is their fated nemesis, her having knowledge of greiver, and even using it against the party as well doesn't make her rinoa. It makes her their fated nemesis. She uses something the party believes in against them. Not a huge leap here. And its never stated that greiver is just a necklace, so it may have well been real all along.
Last point: ulti's time is "far in the future". The age difference between R and U is noticeable, but not "far future" noticeable.