Given the knowledge the Corleones had, if the family had insisted on Michael not being the trigger man, there would have been another way to get the hits done; an arguably easier way.

Have the plan unfold as before - have Michael agree to meet Sol and the Captain, minus the planting of the pistol in the bathroom. Place two unnoteworthy button men in the restaurant; ideally old hands who wouldn't stick out so much. When Michael goes to the bathroom, bada-bing, bada-bang, job done. Maybe Michael gets a little heat for being there but let the pain-in-the-neck innocent bystanders work to his advantage for once.

The novel kinda took on that possibility. Sonny suggested that someone up and coming in the family negotiate with Sol instead of Michael, but Clem said it'd be like sending a rookie to pitch in the World Series. "It has to be Mike," Sonny concluded. Also, the novel notes that Sol had a man in the restaurant watching out for treachery. When Michael asked to go to the john, Sol looked over to the guy, who nodded his ok, as if to say he'd checked things out. I think that guy would have spotted the Corleone men before Sol and Mac entered the restaurant with Michael. Sol would have been looking for that kind of move.

Been a while since I read the novel...what happened to Sol's man in the restaurant then? Why would he just let Michael walk out after blowing a hole in his boss' head?

The novel states that he chose to do nothing as Michael's gun came to bear down on him. He thought Michael still had the gun.