That last set of picture (from diff_lock2) is similar in concept to the hitch assembly I built back in the 70's
on mine I had the same framework on the bike (metal, with a brace to seat post though) and used a CV joint instead of the hinge and angle plate shown. which gave a nice tight 'ball and socket' joint at the hitch. advantage there was that the trailer was free to move up/down left/right but didn't have any tendancy to wander while providing a pretty tight turning radious, and keeping hitch weight low.

Ken.