Like DC, I've always been inclined to give Fredo the benefit of the doubt, and I remain unconvinced that Fredo had as active a role as is theorized above.

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.

As to Tom's admittedly strange wealth of information, I'd chalk that up to directoral license. The movie was long enough without scenes of them investigating Roth and Fredo's whereabouts.