Michael taking off without knowing what happened seems to indicate Buscetta had one job to do, and that was taking care of Roth. If Michael had flied out before the reception, it would single other members in the different organizations that were there, that something was up. Also he had to confront Frado, as well as protect him as he was close to Roth and with Roth out of the way, Frado loses any protection he has. I think Michael was outing to give Frado a pass and put him on the shelf, but that changed when Frado ran away from Michael during the chaos in Cuba.