Would Michael have cared if Rocco had said "It's done Mike" but then proceeded to set up a plan which involved throwaway gunmen, distractions and most importantly had Rocco himself nowhere near the assassination?
If Rocco had strolled back in the compound and told everyone to watch the news would Michael be satisfied that the job was done or be silently asking himself "
I thought I sent that guy on a suicide mission. What's he doing back here? Note to self: watch him..." 
...which raises a question: Since it was important to Michael that Rocco not survive and be a liability to him, how did he assure that Rocco wouldn't do just what you described? When Rocco said, "Difficult, not impossible," we might infer that, as an operating executive of the Family, Rocco easily could have pulled together a hit crew in the manner you described. MIchael would know that. If he'd said, "I need
you to pull the trigger, Rocco--and don't involve anyone else," Rocco would have to know it was an intentional suicide mission.
