Now, that raises a couple of other intereting points (or at least I think they're interesting):
First,Tom surely knew that Michael wouldn't give Fredo a pass, and that when Michael "forgave" Fredo, it was only temporarily until Mama died. Do you think Tom ever contemplated warning Fredo that he was in a trap?
Second, Michael was practically frantic to get Fredo on the plane with him in Havana. But once he was back in Nevada, he just told Tom to "get word" to Fredo, when in fact he could have had Fredo snatched off the streets of NY and hauled to Nevada. Why the sudden shift?
I think it would have been out of character for Tom to contemplate warning Fredo. For all the "non-Sicilian" jibes thrown his way, Tom certainly seemed unmoved when Tessio asked for mercy. His ultimate loyalty, whether out of love, fear, or jealousy, was to Michael.
As to the "sudden shift," I don't find it implausible. In Cuba, Michael had just discovered Fredo's treachery and was, admittedly, heartbroken. I think, in his emotional state, he hadn't decided what to do about it. His urgency was a combination of his agitation, the chaotic scene around him, and his desire to keep Fredo close until he decided on a course of action. Later, Michael has cooled down and his anger has harded, but he still needs to keep tabs on Fredo. So he adopts a different tactic to try to reel him back in.
