Like those witnesses you cite describing slumping on the "first shot" and it turns out they're describing the shot before the head shot.
Like I said, you editorialize. When you say "it turns out" you mean "they really observed what I say they observed, not what they thought they observed"
Kennedy supposedly looking in the direction of Jackie is part of the "severe external stimulus" you cited.
"At approximately Zapruder frame 200, Kennedy's movements suddenly
freeze; his right hand abruptly stops in the midst of a waving motion and
his head moves rapidly from right to his left in the direction of his wife.
Based on these movements, it appears that by the time the President
goes behind the sign at frame 207 he is evidencing some kind of reaction
to a severe external stimulus. By the time he emerges from behind the sign
at Zapruder frame 225, the President makes a clutching motion with his
hands toward his neck, indicating clearly that he has been shot." (VI HSCA 46)
Again, you are editorializing. Nowhere does it say or suggest in that passage that JFK turned toward Jackie, or turned his face toward, or was looking in the direction of Jackie. He moved his head and body to the left from the far right side of the car. Left happened to be in the direction of Jackie. Powers observed it from directly behind.