Everyone made such a big deal about the Nimoy scene, but as i was re-watching it this weekend i realized, Spock Prime didn't tell him anything other than Khan is super dangerous and they were only able to defeat him at great cost.
Then it cuts away. we don't see what he said. He may have then said that's all he can tell him. He didn't necessarily explain in detail how they defeated Khan like many people have inferred.
Plus it doesn't seem like Spock would have needed to talk to Spock prime to come up with the idea to remove the bodies from the torpedoes and detonate them on the Vengeance.