I think we both agree that Michael knew a second traitor was involved -- one whose role was to kill the hitmen at the very least, since Fredo could not have done it. I think we just disagree on the timing.

You believe that Michael knew -- even after Fredo's confession -- that there was a second traitor to be caught. Michael right away would have launched an investigation. If evidence pointed to Rocco, he would have dealt with him long before the Roth hit.

The flaw with that argument is that the movie doesn't show Michael investigating any of his subordinates or even dwelling on the Tahoe shooting. Furthermore, if one believes that Rocco was completely loyal, then Michael would have uncovered the real traitor, since he never gives a betrayer a free pass. We don't see any such punishment meted out.

As Turnbull noted, the Tahoe shooting seems to be on the back burner. Other events are preoccupying Michael: the Senate hearings. his next move on Roth, his mother's death, and particularly his separation from Kay, which brought his anxieties about losing his family to fruition.

This is why I surmise that the thought of a second traitor never even occurred to Michael until he began to exhibit signs of paranoia, with every little perceived lapse in loyalty nagging his thoughts. When he thought that Rocco could very likely be the killer of the assassins, he then "took care of him" via the Roth assassination.