I think the book fleshes out that Sonny was ruled out because he has a wife and kids unlike Michael then but now if he is in the same boat No doubt dilemma!

Sollozzo only wants civilian Michael to proposition but really to buy time until he can finish Vito off
If he did kill Sol and Mac, would he need to go into hiding? exile option is not viable with Kay

Who is going to identify the son of the top Mafia Don