For one, I'm wondering how many traitors there are. While Fredo may have opened the drapes, I don't believe he had the moxy or skill to knock off one or more assassins.

As far as the Miami trip goes, it's all strategy to deceive Roth. It was planned and if he doesn't follow through than he may tip off Roth that he is wise to him. Michael needs a pacific environment in which to carry out his plans. If he goes after Roth in any way, than war breaks out. It takes two to fight and Michael wasn't going to play Roth's game.