Also, doesn't the fact that Fredo says "I don't want you guys calling me anymore" imply that he doesn't want to kill Michael anymore. I think this is one way that people could argue that Fredo WASN'T GOING ALL THE WAY in killing Michael.
Fredo might have said "I don't want you guys calling me anymore" but they DID call him again, and he certainly listened. How else could he have know about Pentangeli and Questadt.
Don Tomasso gave a good summation of what happened next:
After Michael and Tom figure out that Pentangeli is alive and that Michael has been set up in a perjury trap, Michael asks Tom what Fredo knows, and Tom says Fredo says he doesn't know anything. Michael calmly tells Tom he wants to meet privately with Fredo when the great "I'm smaht" scene takes place. During the scene Michael asks Fredo whether there is anything he knows that can be of help, and Fredo says he knows Pentangeli is alive, and then he adds that Questadt belongs to Roth. Seems to me that had Fredo told this to Tom and Michael before the hearings they would have devised some different approach to things, perhaps even outing or taking out Questadt. It also shows that Fredo's treachery against Michael was ongoing because he did not come clean when Michael gave him what apparently was another chance.
