I don't know how tall Secret Service agent John Howlett was, but the 11/27/63 reenactment photo shows him sitting on your beloved box (apparently in its correct position) and assuming a shooting position similar to what the "Oswald" actor does in "The Lost Bullet" for the 2nd and 3rd shots, so I guess it's plausible that 5' 9" Oswald fired those shots while sitting on it with one bun on and one bun off, just like Howlett was doing as he was pretending to shoot.
More importantly, given the fact that we now know that Oswald fired his first, missing-everything, shot at hypothetical "Z-124," (half-a-second before Zapruder resumed filming at Z-133) and we know that the window was only about 1/3 open (going from memory here), that Oswald had to stand and awkwardly lean forward while firing the steeply-downward-angled shot, and that this is corroborated by the fact that in a digitally enhanced clip from the Robert Hughes film which is embedded in "The Lost Bullet," we can see something light-colored (LHO's t-shirt?) moving in the window about five seconds before the first shot rang (or banged, boomed, or popped?) out.
I don't know how tall Secret Service agent John Howlett was, but the 11/27/63 reenactment photo shows him sitting on your beloved box (apparently in its correct position) and assuming a shooting position similar to what the "Oswald" actor does in "The Lost Bullet" for the 2nd and 3rd shots, so I guess it's plausible that 5' 9" Oswald fired those shots while sitting on it with one bun on and one bun off, just like Howlett was doing as he was pretending to shoot. There is no reason to assume the seat box was in the correct position in the Howlett photo. In fact it appears to me to not be correct but slightly further away from the window.
Here is a photo from a similar angle to the Howlett photo that shows a model of the box in the correct position. Compare that with the Howlett photo as you wish.
Here is a couple of photos showing how the sniper could sit straight up while firmly on the seat box. He and his rifle are concealed by the wall and the boxes from Hughes' camera angle.
The next three photos show that the sniper only had to raise the rifle up to his shoulder and aim his three shots. This takes very very little time to do. The three targets are Z160, Z224, & Z313.
Here is an over-the-shoulder photo showing three labeled targets at their proper angles from the position of the rifle in the window.
Any shots earlier than Z160 would have been very awkward but not impossible from a sitting position. Plus, the window box and the metal conduit both become potential interference. I simply do not believe he would have taken an earlier shot intentionally.
More importantly, given the fact that we now know that Oswald fired his first, missing-everything, shot at hypothetical "Z-124," (half-a-second before Zapruder resumed filming at Z-133) and we know that the window was only about 1/3 open (going from memory here), that Oswald had to stand and awkwardly lean forward while firing the steeply-downward-angled shot, and that this is corroborated by the fact that in a digitally enhanced clip from the Robert Hughes film which is embedded in "The Lost Bullet," we can see something light-colored (LHO's t-shirt?) moving in the window about five seconds before the first shot rang (or banged, boomed, or popped?) out.We do not "know" any of those things. Your opinions are not facts. Here is a screenshot from "JFK: The Lost Bullet" showing the sniper's nest window just before the shots. The light colored stack of boxes are in the middle (horizontally) of the window. If the sniper was standing his white shirt would be well above the 1/4 open bottom portion of the window. If he was seated it would be seen in the open portion to the camera right of the boxes. Personally I think I see something in the open portion. But it is definitely not definitive enough to say for certain.