I agree it might seem a bit obvious at first, but its not completely unfeasable that Luca would want to change sides. After all, the drugs business was big money, and everyone knew it. If Luca disagreed with Don Vito's decision to stay out of it, and wanted to make himself a fast buck, its not beyond the realms of possibility that he would defect over.

On the subject of the meeting of the families. It did always puzzle me a bit how Vito knew from that meeting that it was Barzini who was behind the whole thing. During the meeting, I'd sort of assumed that Barzini was acting as a kind of chairman, and that he was trying to mediate between Vito and Tattaglia (although obviously more biased towards Tattaglia). It wasn't really apparent straight away from what he'd come out with, that it was Barzini pulling the strings. But then I'm not a highly experienced and knowledgable Mafia Don, and Vito was!