You can put me in the camp of people who believe that Rocco was simply jealous that Neri had passed him over, and that he was willing to show Mike how loyal he was by going on what was most likely a suicide mission.

If Mike ever suspected Rocco, it was only briefly...during the scene with Tom in the boathouse Tom asks if it could be an inside job, and Mike answers indirectly "all our people are businesmen," thus making ANYTHING possible. My gut tells me he suspected Fredo all along.