I thought the antagonist had been confirmed as Khan for a while? I guess not. I'm not a Trekkie by any means, so I had to look some of this up, but It seems the most "educated" guesses at this time are either Khan or another character from the original series -Gary Mitchell (I had to look him up).
About Sybok- I'm hoping in JJ's timeline he would have been killed on Vulcan when it was destroyed. Although if he was offworld at the time (I cant recall if the location of his banishment has never been stated) I guess it might be possible?
There has never been a "Scientifically Accurate" Dinosaur portrayed on any Television Show, Movie, or Documentary. Only what the human imagination can conceive, and I'm ok with that.