My recollection of the scene is that Rocco Neri and Mike are already in the room when Tom enters, after Michael insults Tom (some say to Neri's pleasure)they get doewn to business. I think it is a given that Michael wants Roth killed, and Tom, as always, tries to talk him out of it by asking whether or not he has to rub everyone out, to which Michael replies, "just my enemies." Then there is some discussion about Roth being turned away by all these different countries, and they conclude that Roth will end up in Miami. Michael simply says he wants the plane to be "met" in Miami. Tom then goes into how this is a logistical nightmare, and Michael responds that if history teaches us anything it is that you can kill anybody. Then he throws the discussion open and Rocco says something like "difficult but not impossible," which Michael takes to be Rocco volunteering for the mission, something many people on these boards was done to curry favor with Michael who seems to have passed Rocco over for Neri. Technically, therefore Michael never gives anyone an order to do anything. If Rocco had been caught and even if he broke omerta, Michael had complete deniability. It is not likely Tom or Neri would also rat him out, and in fact they could say something to the effect that there was some discussion about Roth, and that Rocco somehow got it wrong and went off and killed him. I believe Michael, if arrested for this would have Tom and Neri as witnesses to say "He never gave the order."

Additionally I do not think this is the only time Michael had more than one person in on killing someone. If you look at how he handled Carlo, Tom got plane tickets to Vegas, and Michael plied him with alcohol to get a confession in front of more than one witness that he set Sonny up and that Barzini had approached him. Carlo then goes to the car and there's a driver (forget who and Clemenza in the back seat. Those two had to know what was going to happen. Then as Carlo is garroted Michael, Tom and Neri I think all casually stroll outside to watch. No one seems surprised, so I deduce they all knew what Michael had in store for Carlo.