I agree with Peter Dale Scott on that. The plotters' hoped that Johnson would decide to invade Cuba in retaliation for the assassination. (Or even a first nuclear strike against the Soviet Union, at a time when it was thought that America would prevail.) This is Phase 1.
But if Johnson chose to reject the Cuban/Soviet evidence, the plotters wanted to make sure that the Johnson Administration could blame it on Oswald alone. The reason being so the investigation would focus on Oswald and not on identifying the real plotters (CIA elements). For that purpose, the plotters made arrangements in advance to alter photos and films as necessary and as possible, and to control the autopsy. All so that the evidence would indicate gunshots only from the rear.
That explains how the medical and photographic coverup could be done as quickly as it was! (I used to believe that this was a part of the LBJ/FBI coverup, rather than part of the plotters' plan. But that idea made no sense because the LBJ administration couldn't possibly have acted so fast.)