Without going into a point-by-point, one main objection is the issue of the $2M. I think we all agree that Roth's motivation in keeping Mike alive was to get his hands on it. But, if you believe that Fredo was an active participant in the entire plot, then Roth, in effect, already had his hands on it.
Fredo surely would have told Roth that he was coming with the $2M. In fact, he probably did, as Roth's "sixth sense" told him Fredo and the money had arrived.
If Fredo was truly in deep with Roth, Roth would have killed Michael as soon as Fredo had the money. There's no way Fredo would have given up possession of it to Michael.
As to their escapes, there were any number of ways that Fredo and Roth could have gotten off the island; both were clearly well-connected in Havana.
I have to disagree LW with some of the points you raise. First of all, from Roth's perspective it is not clear when he knows for sure that Michael Corleone is on to him. The whole subtext of the Michael-Roth relationship is the two men circling each other, each outwardly flattering the other but inwardly suspecting the other of treachery.
As far as Roth Killing Michael as soon as Fredo had the money, I think he had to go through the charade of Fredo bringing it to Cuba and having Michael giving it to him. Having missed his chance to kill Michael in Tahoe, Roth had to wait and see if Michael would personally hand him the money or ultimately bail out, as he eventually did. Roth's plan was to have Michael killed by Batissta's military after the New Year's party, and at that point the money would either be in his hands or in Fredo's after Michael's death. Fredo would never openly defy Michael by giving the money to Roth while Michael was still alive. At the heart of Fredo's treachery, at that point in the story, was his keeping it from Michael, not putting it on display.
As I said earlier, I think Roth's going to the hospital was a ruse to protect himself, because after the scene where they confront one another about Pentangeli and Moe Green, Roth has a pretty good idea that Michael knows he tried to kill him, and that Michael is about to try to take Roth out himself.
Finally I don't know that Fredo was all that well connected in Havana. He does say it is "his kind of town," but in the Superman scene he implies that Johnny Ola was the one who knew the town like the back of his hand, and not himself. Besides, how well connected in Havana could a guy who didn't know how to say "Banana Daquari" be?
