Good point, Olivant. As you know, in the novel Sol tells Tom that he specifically ordered his men not to harm Fredo--"Fredo is alive because of me." Tells me he had discounted Fredo even before the bullets started flying. Fredo acting competently would have thrown Sol off.
The only reason Fredo is there driving his father because he is not compotent. If Fredo had a more important position in the family, I would doubt that they would both (he and Don Vito) would be left out in the open.
The Turk not killing Fredo served several purposes. Yes, it is probable that Solozzo discounted Fredo as a factor, but he also did not want to "drive" the Corleones "over the top," however the situation might have been different if Fredo wasn't the one driving the car.
