I think that the Fredo-option is the most likely one. First because this is the only alternative that we are given hints for, like the real drink scene.

And while reading this thread, another thing came to my mind. It is true that Fredo didnt want anything to do with Johnny Ola and Roth after the Tahoe assasination attempt. But remember that Fredo knew that they got Pentangeli. This could mean that he had been further cooperating with them. Or maybe they used him again without his knowledge, because after all, he was weak and stupid...