The picture I posted of Specter holding the rod at the approximate trajectory of the single bullet shows the trajectory works without the jacket being bunched. A white chalk mark had been placed on the agent's coat 5 1/2 inches down from the top of the collar. Specter held the rod to the right of that spot. The rod passes to the right of the knot in the agent's tie and on to the entry point on the back of the agent taking the place of JBC.
Specter didn't hold the rod at the level of the chalk mark. As you can see below, the rod was somewhat above where the chalk mark on the jacket was.
What that picture also shows is the natural slope in a man's upper back which I estimate to be about 45 degrees for the agent pictured. The geometry is such that the vertical separation between the entry point on the back and the top of the collar is less than 5 1/2 inches. Those who remember their high school geometry can figure that the 5 1/2 inches is the hypotenuse of an equilateral right triangle and the square of the hypotenuse of a right triangle is equal to the sum of the square of the two adjacent sides. 5.5 squared is 30.25. The vertical separation between the top of the collar is the square root of (30.25/2) which comes out to 3.89 inches rounded. Of course, this is an approximation because I doubt the slope of that agent's upper back is exactly 45 degrees.
What that picture shows is a fairly good representation of the path of the bullet through Kennedy. Not perfect though, But it does show that the vertical angle of single bullet trajectory could work
There are pictures taken during the course of the motorcade show that at some points the jacket is bunched and at other points it is not. The image below is the closest I can find to the time of the single bullet and it does not appear to me that JFK's coat is bunched at the top.
https://texashistory.unt.edu/ark:/67531/metapth1352810/m1/1/high_res/
The slope in JFK's upper back looks to be a little less than 45 degrees, which changes the calculation but a I think we can safely say the vertical separation between the top of JFK's jacket is less than the 5 1/2 inches.
There are numerous photos of Kennedy throughout the motorcade that show his jacket to be bunched up. The photo that you linked to is of rather poor quality. The Croft photo is the closest to the time of the single bullet shot in which the bunchup can be seen. You can't see it the best but it's there.
https://www.jfkassassinationforum.com/Gallery/Croft%20Gallery/croft~0_small.jpg
