Three questions:
1) What was Barzini's strategy when Sollozzo went (or was sent) to the Corleones? If Sonny hadn't made his mistake, there must have been another idea in Barzini's mind. And what if Vito had agreed?
2) At what time did Barzini talk to Bruno Tattaglia? The novel tells us that after Sollozzo's visit Luca went regularly to Bruno's bars. Was Bruno instructed by Barzini in advance? Or did he call Barzini: "Hey, that old Luca is hanging around all the time in my bars, what shall I do with him?"
3) If Vito hadn't been slippin' and not sent Luca, what would Sol have done to stop that killing machine?