...The novel makes it clear that Vito was heavily involved in planning it.
So does the film, to a lesser degree but just as obviously. Again, nobody is denying this.
... Apple is convinved that Vito would have had to murder one of the Dons himself or to have directly ordered one of them murdered to have broken his word.
Not at all. But he would have had to: 1. Meant for it to happen before Michael was set up for elimination, and 2. Been alive when it happened. Neither was the case, and therefore he did not break his word.
...Vito intended to murder Barzini and Tattaglia in good time and his planning of it breaks his word.
No, it doesn't, because the planning of the murders was in preparation for the offensive on Michael which was certain to come.
...Tessio's betrayal was an aside....
It wasn't. Vito had warned Michael of the traitor that would approach him with the meeting. That traitor turned out to be Tessio, who had conspired with Barzini to move in on Michael. The existence of the traitor AND his murder along with the rest would have been an integral part of the plan. It was just a matter of waiting to see who it would be.
That's all.
