Going back to Fredo's escape from Cuba, I am not so sure Roth would have taken him out. In the first place by then Roth knew that Michael was aware that Roth had organized the hit, and it would have been a small leap for Michael to figure out Fredo had been the traitor in the family. With Roth's assassination plan gone to hell with the Castro uprising, Fredo was of no more use to Roth, and it would make more sense that he be eliminated.