Item #3: Could it be that LHO did practice with the rifle, adjusted it the best he could, made a mental note that it was just a little off target one way or another, and then he could adjust his aim a little to compensate for this?
Sure. Do you have any evidence that he ever practiced with this rifle?
Item #4: As I remember, the disassembly needed (so that the rifle would fit inside the bag) involved removing the wooden stock from the metal barrel.
Or maybe that rifle was never in that bag.