Probably Michael's luckiest break (other than ducking the bullets in his bedroom) was Batista's abdication. Sure, he counted on his bodyguard killing Roth before he left Havana. But, since Roth had already arranged for Michael to be killed on his way back to his hotel after the party, I'm sure there were SIM (Batista's secret police) agents bird-dogging him at the party. They wouldn't have had any connection to Roth--their orders would be to make sure Michael got killed in that military car, and they would have followed those orders. Batista's abdication made them run to save their own asses.