I agree the characters are not spot on but I think the movie is great. The Valeris mind meld was a very powerful scene indeed but I don't understand why Spock pushes for the location of the peace conference when immediately after the mind meld he just contacts Sulu anyway! It was not necessary to forcibly try to find that information.
Yeah, the the fact that Spock says to contact Sulu and ask for the location of the conference seconds later sort of undercuts the necessity for the forced mind meld and makes it look worse.
However, perhaps it was the mind meld that clued him in that Sulu would know the location of the conference. Like for instance Valeris knew the sector the conference was in but not the exact location, and Spock knew Sulu was in that sector. Or Valeris knew the ships that were assigned to patrol the region around the conference (without knowing its precise location), and revealed that Excelsior was one of them. Those are both pieces of information one of the lower level conspirators like her might know.