Oh it goes without saying that Fredo is a traitor too (or is the only traitor, depending on one's theory). He's certainly the primary traitor. I believe he is duped by Roth into doing some things that would make the hit on Michael possible. (I also believe that Fredo had no idea that Roth's men planned to kill Michael.)

I think Rocco could have been the person who killed the two gunmen.

Pentangeli wasn't involved at all. Michael believed it was either Roth or Pentangeli who arranged the attack. He learned very soon that it was Roth alone.

And now I have a Michael-like hunch that a moderator will soon come around and move the posts we've been exchanging into a new topic called "Traitors" (or something like that).