Good questions, Oli. It didn't take genius for Michael figure that Fredo was too weak and stupid to run the family in his absence. But he didn't figure that his constant sidelining of Fredo into "Mickey Mouse" jobs (and his put down of Fredo in Vegas after the Moe Green meeting) would cause enough resentment to turn his brother into a traitor. Fredo might have come to Havana on his own and known how to show people a good time. But, I thought it was suspicious that Johnny Ola said, "We never met." If Ola remembered Hagen from the old days, he surely must have met Fredo as well. Why didn't Michael pick up on that? For that matter, Sonny should have picked up that Carlo would be filled with enough hatred of him after his public beating and humiliation that he'd turn traitor and set Sonny up.
As Vito said in GF: "Women and children can afford to be careless. But not men."