I think you're dead on the money in suspecting Rocco. Note in addition that Rocco is wearing an orange jacket at the beginning of Pt. II, and that it is Rocco, not Ola, who hands Michael the "orange from Miami."
A traitor on the inside must have killed the two men sent to kill Michael, and once it is granted that it could not have been Fredo, all arrows point to Rocco.
