I don't see why that speech couldn't have been orchestrated by Barzini who virtually controlled the Commission by that point. He could've said to Zaluchi that "drugs are going to happen whether you like it or not, perhaps you can help me convince Don Corleone." The problem is that the film doesn't highlight any such prior relationship between Corleone and Zaluchi.
The speech by Zaluchi in the film is taken directly out of the novel. If it was orchestrated by Barzini, Puzo would've mentioned it. Puzo states that "Zaluchi disapproved of traffic in drugs" and that Vito "could count on Zaluchi for support."
Doesn't mean that you're theory isn't plausible though.
