IF Rocco was involved and Michael knew or had any inkling about it... I believe Michael applied "friends close, enemies closer" kind deal in regards to keeping Rocco around(similar to Roth and Michael)... AND... the reason why Michael didnt fear Rocco trying to kill him after the failed hit was because... Rocco was a businessman... the business "deal" had changed when attempt failed.... AND I believe that he allows Rocco to redeem himself with Roth assassination/kamikazi hit