Dan-
Thanks again for reading, and I enjoy your observations.
Yeah, that is a good point: Why not just take a real shot at the President from the Grassy Knoll-pergola area and then run away. It would be both a diversion from the main assassins and another chance to kill the target.
Well, I think a few reasons.
The limo is running left to right from anyone standing in the Grassy Knoll area (the Zapruder film view, for example). If one wishes to remain hidden from view, that means crouching down behind the pergola or the stockade fence (remember Secret Service eyes forward from the motorcade).
A shot at the horizontally moving target with a handgun is possible, but probably not reliable.
So a sensible to-kill Grassy Knoll shot means pointing a long rifle at the president, but no credible witness saw a rifle that day near the Grassy Knoll. None has shown up on photos, or movies. None was found tossed aside after the event. Some have pointed out that a shooter behind the stockade fence has his view of the motorcade blocked by the pergola-monument "until the last moment."
I can imagine carrying a break-apart rifle to ground-level along motorcade route. But how to run away carrying a rifle? But no rifle found at the scene.
So I deduce no rifle was used on the Grassy Knoll.
Another thought: The real assassins are hired and they survey the scene. They are in the Dal-Tex building, and they are not affiliated with Oswald. They say, "Nice layout, we are shooting from behind the president, and he is more or less moving in a straight line from us. But we need to escape after the act. So plan a diversion near the Grassy Knoll, but wait until our first shot."
The Dal-Tax shooters have semi-automatics, so it will be fast after the first shot anyway
Also, not every plan is a perfect plan. Maybe the diversion was a "bad idea." It risked exposure before the real shots were fired. It risked capture of the Grassy Knoll diversionist, who might confess. The more confederates in a conspiracy, the more chances for leaks, bumbling, extortion (one might wonder how long the diversionist lived after Nov. 22).
But the "Grassy Knoll" diversion plan fits the events. It was the plan they went with. People make plans, and plans have flaws. They have to work with available resources, the topography at hand, and personnel. Might have had only a week or two for on-the-ground for planning. They planned a diversion.
Anyway, that is my reasonable conjecture on the Grassy Knoll smoke-and-shot show.