Sorry, but you haven’t studied the head wound and the force and direction of skull fragments per testimony without film. Indicates a shot from the knoll fence area. You are correct that shot straight was ideal, but they knew rifle was bad and they needed it to point to oswald. Book bldg would have been a poor retreat for real shooters.
The extra time only decreases the need for expert marksmanship. I have looked at the head wounds as represented online and do not find them conclusive in terms of projectile force and direction. There was immediate eyewitness testimony concerning a rifle barrel protruding from an upper window in the TSBD. I will grant that the proposition of additional shooters is not preposterous. At the same time, neither is the proposition of Oswald as a solo act. A jump from 6 seconds to 11 in the time it took to fire the shots only strengthens the latter thesis.