A very interesting theory, jrp. Thanks for posting it.

But, I agree with Oli for the reasons he stated.

Complicating this a bit is the timing: On learning of Sonny's death, Vito immediately tells Tom to arrange the Commission meeting, yet in the sequence of the film, Michael learns of Sonny's death, then arranges to move to Siracusa, then the car is blown up, no doubt on Barzini's orders. So: did Barzini move on Michael (and/or Apollonia in your theory) in Sicily after being invited to the Commission meeting--to further weaken Vito's bargaining position? Or, perhaps, did Barzini reach out to Fabrizzio before Sonny was killed--and it took that long for Fab to find an opportunity to make his move?

Barzini and the other Dons saw the Commission meeting as the opportunity to get Vito's police and political protection for the drugs trade. If Barz had intended to kill Michael after being invited to the Commission meeting, Vito would never have forgiven him--and that would have been the end of any agreement on drugs. That view supports your view that Apollonia was the target. But, singling out Apollonia would have been very complicated, and as we saw, her presence in the car after the bomb was wired was pure chance.