I wondered about that scene too, Turnbull. Even Tommasino believed that the local hoods in the area were after him...until, he says, they believed Michael was dead. They quit coming after Tommasino (and by extension, Michael/Apollonia) after that. The smokescreen was no longer needed. All of them believed that Fabrizzio meant to kill Michael, and that Apollonia was in the wrong place at the wrong time. That's where I differ.

From Barzini's perspective, Michael was on ice in Sicily. He wasn't going anywhere, nor doing anything, and Barzini apparently had contacts on the ground in Sicily to monitor him. For all intents and purposes, Michael was out of the game entirely. What would be the benefit of Barzini leaving Michael on ice in Sicily, then calling a hit on him before Vito called the meeting? Or calling a hit on him at all? A hit on Michael in Sicily, before or after the meeting call, would have the potential of causing Vito to go to war.

Chronologically in the movie, the hit goes down after Vito gives the order to Tom to contact the other Dons for the meeting. By then, Barzini would know that whacking Michael could cause Vito to declare full out war. Even if Barzini won the war at that point, the Corleone family would be decimated and he would only have the bones left to pick. Not to mention the damage to his own family, and increased heat from the law. I can't see that Barzini would come out ahead in such a scenario.

Between the distance of the car from the villa, Apollonia's known presence in the car and Michael being nowhere near the car at the time of the blast, I can't see any way that Fabrizzio's plan (if he's aiming for Michael) would've been successful. In addition, Fabrizzio was ordered by Michael (in the movie only) to take Apollonia to her father's home. Why was she not by his side as he left instead of in the car?