To me, Brennan is talking about the two incidents when he saw the gunman. Not that he saw the gunman "from his belt up" all during the shooting viewing, which is your interpretation.
What do you mean "
all during the shooting viewing"? How long do you suppose the "firing of the gun" took? Remember, Brennan only claimed to see the head shot being fired (though he didn't actually see any discharge). You seem intent on combining "after the shot was fired" with "at the time he was firing".
The gunman was leaning forward in order to use the top of the box (Box "A") to steady the rifle. After the last shot, he straightened up his spine or stood up altogether. Either would expose more of the gunman to Brennan's view. Where do you get "crawl"?
Then what do you suppose Brennan meant by "stepped down"?
That would be you promoting Brennan as a "loon" because you figure he saw the gunman from the belt up all the while during the shooting.
I didn't call him a loon, though his various accounts are contradictory and unreliable. He supposedly saw both the shot being taken and JFK's head explode. But I'm not "figuring", I'm looking at what he said: "
at the time he was firing the gun, a possibility from his belt up".