Otherwise if Mama had already been gone when Michael found out that Fredo betrayed him, Fredo wouldn't have ever even made it out of the boat house alive.
If Mama Corleone was already gone, Fredo may not have made it out of Cuba alive.
No way Michael viewed Rocco as a traitor. He never would have kept him around so long. Once Fredo gave it all up in his boat house "confession" there would be no reason for Michael to keep Rocco alive, if he suspected him.
"I got news for you. If it wasn't for the toilet, there would be no books." --- George Costanza.
