The entire conspiracy and cover-up would have been air tight but for the shot or shots from the grassy knoll (Holland, who was very familiar with guns and gunshots, heard two different gunshots from the grassy know, a rifle and a pistol). Had there been no shot from the front, no one would have questioned the Oswald story.
Have to believe the plan was not to use any shots from the front unless the shots from behind didn’t kill Kennedy, which is what happened — the one shot in the back from behind was too survivable and another shot from behind missed completely.
The shot from the grassy knoll was the one fly in the ointment they couldn’t erase. The image of all those people rushing up to the spot where the shot (or shots) on the grassy knoll came from — not even the corrupt Warren Commission had the power to eradicate that fact.