Thanks for posting the results of your experiment. Yes, I agree the towing forces ought to be taken by the square members of our draw bar and receiver. On my tow vehicle, I have too much slop, and the pin gets a lot of the load. When I hand lift my stringer, my pin does not rotate. When I twist my stringer, the pin does not rotate.
I will get a 2.5 inch square draw bar from Propride eliminating the sloppy reducer sleeve. And I will shim or clamp the draw bar to the receiver eliminating as much free play as possible. And I might make a precision hitch pin out of tool steel that is about .660 in diameter taking further slop out of my hitch assembly.
I want the square members to take the load.
I read with interest the thread about the swaying car dolly in this forum and all the movement noted in the hitch. These things are built sloppy.