If Itachi had been the Hokage, would the whole Uchiha Clan massacre have happened? And would it have been a good choice? I personally think that Itachi would have made a pretty good Hokage.
Hard to say, depends on how the Uchihas would have reacted. Would they try to take over and gain power through Itachi or would they have been happy with Itachi as Hokage since it shows that Uchihas do have power? Also why would they make Itachi Hokage? I mean who would elect him at 13 and for what reason? They could easily vote for Kakashi who has more experiences, is older, graduated from the academy at 5 and became Chunin at 6, have fought in war and have earned the respect of others in the village.
You have to think about that stuff. Yes Itachi would be a good Hokage but we don't know what kind of changed would happen to him since he may not be the same as we know him, he was a pacifist but also could walk the dark path if he needs to be. Would Itachi be influenced by his clan in his ruling?