Not bad, Cristina. You make some compelling points. I especially like the idea of the drainage ditch as the escape route.

However.....

Had Fredo known of Rocco's complicity he certainly would have told Michael during the "I didn't know it was gonna be a hit" scene. Why would he take the fall himself?

Even if Fredo didn't know that Rocco was involved, as dumb as he was he still would have realized that, given the death of the hitmen, he was not the only one involved on the inside, and offered some theory to Michael as to who else had participated.

Even without any disclosure by Fredo, I have to give Michael credit for being able to figure out that another or others on the inside were involved besides Fredo. He let Rocco stay as a member of his inner circle for an awfully long time, which leads me to believe that he didn't suspect him.

As flawed as the film is with respect to this particular (and important) aspect of the plot, I believe that if it was the intention of MP and FFC to have us believe that Rocco was a traitor we would have been given some concrete evidence of that somewhere along the way.

Fredo was the only one identified as a being traitor. I believe that it was the intention of MP and FFC to leave us thinking that it was Fredo who killed the hitmen, without realizing that some 30+ years later there would be fanatics like us still trying to figure the thing out.