Thank you!

Barzini tells Tom "the Don rest in peace was slippin'. Could I have gotten to him five years ago?"
Sorry, dontomasso, it was Sollozzo who made that quote.

I say all this because Turnbull's analysis overlooks the fact that "it was Barzini all along."

In another thread that you commented on, I theorized that it was Barzini all along--but that he kept his hand hidden by encouraging Sollozzo to cite Tattaglia as his backer. Tattaglia, the pimp, would be less threatening to Vito than the more ambitious and capable Barzini. And in other posts, I suggested that Vito's biggest mistake was not realizing that even he couldn't forever prevent the drug trade. I did say elsewhere that he could have temporarily headed it off by arranging for Luca to whack Sollozzo under a pretext and making Tattaglia suck it up. Barzini then probably would have withdrawn, but only temporarily; and would have found another opportunity down the road.
But, to your overall thrust:
Puzo's on your side. One of the most famous quotes in the GF novel is Vito's, "A man has but one destiny." I posted this thread to show that, at least theoretically, Michael had free choices at every stage of his life. But if you believe that crime was his destiny, he had no choices. All of his pitiful attempts to rationalize his "legitimacy" were the work of a criminal mind.