The tidbit that started this idea growing in my mind was the order that Michael gave to Fabrizzio after he asked if Michael would be driving himself and if his wife would be coming along: "I want you to take her to her father's house until I know things are safe." To me, that implies that the car is for Michael, and Apollonia is to go with Fabrizzio on foot, by burro, whatever he can find.

If Fabrizzio's orders from Barzini were to assassinate Michael, then Michael just handed him the perfect alibi on a silver platter: he was with Apollonia on a journey to the Vitelli home when the car exploded.

It hinges on the time between Vito giving the order to Tom to arrange the commission meeting and the actual meeting itself. Tom would have to coordinate the schedules of multiple Dons as well as allow for travel time for both the Dons and the Bocchicchio hostages. If all of that takes a week, that's plenty of time for Barzini to phone it in to Sicily. Thus, Barzini put his plan into motion after the meeting was called; he surmised that Vito wanted to bring an end to the war and to get his son out of Sicily.

Why would Fabrizzio leave Apollonia in a rigged car if it were intended for Michael, and if she was to go to a different destination within walking distance? Why leave the car parked so far away from the house, particularly with the keys in the ignition and in the presence of a girl who's always super eager to drive? Why did he not get her out of the car and get out of there as quickly as possible after arming the bomb?

Something else too: we learn later in the novel (and a deleted scene from GF2) that Fabrizzio was extracted from Sicily after the assassination and was set up with a pizza shop, courtesy of Barzini. If Michael was the real target of the attempt, Fabrizzio failed miserably. Not only did the bomb get the wrong target, the car was parked too far away to even get Michael in the blast radius. Michael went onto security lockdown at that point, so no further attempts would've been possible. I don't get the impression that Barzini would've rewarded Fabrizzio's failure; I think he would've left Fabrizzio high and dry in Sicily if he had failed his mission like that.

My thought is that Barzini's only move, since he can't whack Michael outright due to the likelihood of Vito declaring war, he moves to break Michael's spirit, to send a message. Strike as close to home as possible.