Something else sticks in my craw re. the Havana scene:
In the boathouse confrontation later, Fredo says, "I swear to God I didn't know it was going to be a hit. Johnny Ola bumped into me in Beverly Hills...he said there'd be somethin' in it for me..." Sounds like the kind of simpleminded, impulsive, greedy move that a dunce like Fredo would have taken, without thinking of the consequences.
But then I'm reminded of the Superman scene in Havana. How did Fredo find the place? "Johnny Ola brought me here...old man Roth would never come here but Johnny knows these places like the back of his hand." So now I infer that Fredo didn't just "bump into" Ola in Beverly Hills. It wasn't just some spur-of-the-moment decision. Evidently he was, unbeknownst to Michael, meeting with Ola (and probably Roth) in Havana previously. What were they doing in Havana--tarpon fishing? Visiting sex shows? Or, were Ola and Roth showing Fredo the empire that Fredo could run once his brother was out of the way?