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.
There are a lot of good points in this thread. Nicely done, TB et al.
I can't address each of them for the post would become an essay.
However I just can't see Fredo having an active or important role in the conspiracy to kill Michael in Tahoe. I don't think Roth or Ola would have entrusted him with the details. I also think that while Fredo was quite happy to be doing what he thought was important business behind his brother's back, ultimately he wouldn't have sanctioned Michael's murder.
The late night call from Johnny Ola and the aborted heart to heart over the banana daiquiri confirm that for me. Fredo got in over his head, used by people who played on his jealousy and resentment. A useful idiot.
The seeming reconciliation between the two brothers also makes me think that Fredo was unaware of all of Roth's plans. Fredo won't let go of Michael; he hugs him fiercely and with abandon. If Fredo had honestly wanted his brother dead, why would he have bothered to be so demonstrative. He was seeking forgiveness for his mistakes.
I think that Roth did have an organization. The character was based on Lansky, who also had an organization. It wasn't as flashy as some of the other families but it was , at least during the fifties and sixties, as powerful as some of the better known families.
Roth probably didn't have the muscle to go toe to toe with the Corleones (either in NY or Tahoe) and that wasn't his style anyway. Hence the long drawn out seduction of Fredo which leads to an attempt on Michael's life and the low level sniping between Pentangeli and the Rosatos in NY. Roth hoped to keep Michael off balance dealing with internal matters long enough to get the $2 million investment and have Michael assassinated and rebels blamed.
Roth would then have gotten more money for his casinos, protected his interests from an aggressive rival and avenged his buddy Moe Green.
Well it was a small essay anyway...
