TB you bring up a provocative point in saying that Roth had no organization. Assuming that is the case, then it is likely that Roth contracted with the Rosato Brothers to execute the hit on Michael, and that the assassins got on to the estate either as party crashers or with Johnny. This is likely because the Corleone people assume the contractors were "hired out of New York." It is conceivable that Fredo had a role in hiding,and maybe even killing the assassins. In the movie there is a brief scene of Deanna shrieking that there are two of them "lying dead outside my window." This would indicate that the shooters were told to go to the cabin or whatever it was in which Fredo was staying to receive further orders, when in fact the plan was to keep them quiet forever by killing them.

Additionally, Fredo knew that Frank Pentangeli had come all the way to Tahoe to talk to Mike about Clemenza's demise, which Willie Cicci was quick to say "was no heart attack." Fredo would have passed the information to Roth that Pentangeli thought the Rosatos were somehow responsible for Clemenza's death, and that he had gone out of his way to appeal to Michael to allow him to keep the territories Clemenza had promised them. Roth would not have known the outcome of the discussion, but when Michael came to visit him and tell him Pentangeli had orchestrated the hit, Roth has a virtual green light to take Pentageli out. At that point Roth thought he was in the cat bird seat because he had heard from both Corleone brothers that there wasw a rift between Michael and Pentangeli.