I agree with all here:

Both Rocco and Neri weren't beyond suspicion of being complicit when Michael left Tahoe.. As Michael told Tom, right after the shooting: "All our people are businessmen. Their loyalty is based on that." Also, by not taking them to Havana, he was signaling Roth that he hadn't yet identified the traitor. Fredo's gaffe at the Superman show revealed his treason. Technically, it didn't totally dismiss the possibility that Rocco and/or Neri were in it with Fredo. But, I think Michael knew that neither of them would have been stupid enough to side with a dunce like Fredo--or that Fredo would have had the coglioni to approach them.

I also agree that Buscetta must have been part of some old Sicilian loyalty connection that Vito passed along to Michael before he died. He might well have been connected to Don Tommasino.