Much agreed.
Beyond that, I strongly suspect even if JFK had reduced the number of boots on the ground sometime after 1964, he still had other means to help SV, such as naval blockades, air wars, war materiel, money assistance, signals and other intel, diplomatic pressure and so on.
JFK was instrumental to the founding of SV (more so than any other American), and believed deeply in thwarting communism. He wasn't just going waltz away from SV.
James Douglass' book is just fantasy.
The ideology writes the agenda, and the agenda writes the narrative.