TB, you hit it on the head. To properly interpret the movie, we are forced to remember the novel's content. In it Vito told Michael that he probably didn't have the heart anymore to do what had to be done and that's why he was retiring. Vito ended the war, period. He had formulated a long range plan for revenge and murdering Carlo would have tipped his hand. That Carlo would smack around the daughter of a Don is sufficient indication that he was quite a stupid person. That he allied himself with Barzini is a gamble, at best. So, it's no surprise that he didn't appreciate the threat to his life if he played a part in Sonny's death.

And Henry, I'm not sure why you find it to be such a mystery why the gunmen were waiting at the tollbooth. That's what they were instructed to do.