I'm not sure why Michael had a plane ready to fly him out. He knew about his and Roth's intended assassinations, so why did he attend the reception? He couldn't know that Castro was on the doorstep nor could he have known that Roth's murder didn't take place. However, he should have realized that once in the Palace, Batista could have had him locked in. Why not fly out before the reception?
Michael knew that he had to have Roth killed before he (Michael) left the Presidential New Year's Eve party. He also knew that SIM ("Batista's Gestapo") would regard him as the prime suspect when Roth's body was discovered--because Roth had engaged SIM to have Michael assassinated following the party. Michael's attendance at the party was his attempt to establish an alibi. Of course, SIM would pay no attention to legal or diplomatic niceities if they'd grabbed Michael. So, he had the plane on standby to get him out quickly.
I think he was counting on just enough delay in the discovery of Roth's body, and general New Year's Eve street mayhem, for him to be seen at the party and to get away to the plane inconspicuously. Once he was back in the US, the alibi would help him if the Cuban government demanded he be extradited for Roth's murder.
