re: Low probability branches
I'll grant your first couple of points, about the close balance of opinion, and about the Bay of Pigs precedent. But I tripped and fell over this:
HERE'S the scary part. The Soviets had tactical nuclear weapons in Cuba for that eventuality. AND THEY WERE UNDER LOCAL CONTROL. The local commanders had full authority to use these weapons if necessary.
The part about local control contradicts everything I've ever heard about Soviet military doctrine. The chain of command was everything, initiative was discouraged (often with a bullet to the head).
Not that the Soviet Cuban brigade was cut off from Moscow. Rather, I think that any rash impulse to use tactical nukes would have been dampened by the need to consult with Moscow.
Remember that although in this scenario we're substituting Nixon for Kennedy, all the other players remain the same. Would Kruschev have pushed Nixon any harder than he pushed Kennedy? Would Castro have been any more willing to let Moscow sacrifice his country in their game? Or would the "inexorable forces of history" have rendered our change in Presidents moot? It happens frequently in alternate history stories that the timeline "repairs itself" to produce the same outcome despite changes in events.