A potentially fatal flaw and loose end in the Corelone plan was that unarmed Michael was a sitting duck, vulnerable to Sollazzo's presumably armed driver outside.

Sollazzo's driver mysteriously and conveniently disappears when Michael exits the restaurant. This is true both in the book and the film. Puzo and FFC gloss over this. Maybe the guy just went around the corner for a smoke?

There are no other Sollazzo people outside (book and film). In the book, there is a Sollazzo goon inside the restaurant, but not outside. If we assume that Sollazzo people were outside, we must also assume that Corleone soldiers took them out for Michael's safety.

However, during the Coleone planning sessions, neither Sonny nor Tom nor anyone else mentions whacking potential Sollazzo people outside the restaurant, in defense of unarmed Michael.

Sonny tells Tessio to pick up Michael after the job. That's it.

Michael was just lucky.

Here is Michael thinking... "Gee.. I just whacked Sollazzo and McCluskey. Now, I've got to drop the gun. But what about LOU THE DRIVER outside? He must HAVE A GUN! I better hold on to my gun just in case.. But no. I can't get caught with the gun. Maybe Tessio will take care of the Driver. But Tessio is such an old fart? Sonny never ordered Tessio to do this!!!.. What do I do.. shit in my pants... MARIO PUZO, please write in the story that the driver disappears... .... ... ... Thank you Mario!! Now I can drop the gun and leave the restaurant.. I'm the most important character in the trilogy, you can't kill me this early."