I am reviving this thread because it is such a good one.

The real issue here is that Barzini wanted into the Narcotics business as did the heads of the other families. Only the Corleones did not want to go along. Sollozzo was brought in by Barzini to do the dirty work. After the meeting at the Genco office, Sollozzo would have had to go back to Barzini to tell him the news, and there is no question that he would have had to get permission to stage a hit on the Don (Barzini tells Tom "the Don rest in peace was slippin'. Could I have gotten to him five years ago?" This is an acknowledgement that Sollozzo was far enough down the food chain that there would have been no way he could have tried to kill the Don before.
I say all this because Turnbull's analysis overlooks the fact that "it was Barzini all along." Let's say the Corleones put out the word about the dirty cop, and got Sollozo arrested. This would simply have put Barzini's plans on hold until he could get another "top narcotics man" to replace Solozzo, and to introduce the drug trafficking. From the Corleone's vantage point at the time, it was Sollozzo, not Barzini who presented the immediate problem, but in the long run, Michael was right that the "key" to everything was killing Vito one way or another. He may have been right for the wrong reason, but he was right nonetheless. It is logical that the other heads of the families were trying to avert an all out war after Sonny hit Bruno Tattaglia, and they made the deal to have Mike meet with Sollozo and McCluskey. It was here where Michael crosses the Rubicon, because he knows that only he can get close enough to both of them to shoot them dead.
Once Mike gets back form Sicily, he has lost his wife to the "war," and he has been mentored by Don Tomassino, who was a great influence on his life. I believe Michael thought he could turn the family legitimate, but he eschewed legitimate means to reach his goals -- as Kay reminds him in GFIII his big thing is "reason...backed up by murder." His tragic failure was that he did not understand that the only way his famly could become legitimate was if HE became legitimate. This moral confusion is in evidence in the conversation with Sen Geary when Mike says "We're both a part of the same hypocrisy, but never think it applies to my family."