While I'll never believe that Carlo was not part of the plot, I do appreciate the logic of your reference to Vito's promise. However, to this I would point out the following: the Turk says to Michael in the car, after Michael says he doesn't want his father bothered anymore, that "I swear on my children he won't be." This was a lie, and in the restaurant the Turk admits as much: Michael says he wants a guarantee that there will be no more attempts on his father's life, and the Turks responds, "What guarantees can I give you, Mike?" (In the novel, if I remember correctly, Puzo writes that at that point Michael knew for sure that with this little meeting Sollozzo was just trying to buy time, and that he would try to make another attempt to kill Vito.)