Sorry for the delayed response Allan.
Can't tell if the shocks make it better as I've never noticed it before, but I doubt they would have much impact as the suspension would not be working very hard like if the van was being buffeted laterally by wind or bouncing and pitching on a poorly made road.
Going back to the other point you made, I always believed the law went on the gross train weight stamped on the towing vehicle's plate, so it would not be illegal to tow over 100%, just inadvisable. At the other extreme, I'm sure my brother-in-law's Chrysler Voyager can't even tow 85% before it's exceeding the GTW (!!).
Another thing I notice towing my van, that when going in an absolute straight line, in my o/s mirror I can see down the length of the offside of the van including the rear corner, but in the nearside mirror I can't see the side or rear corner. It's as though the van body is not sitting squarely on the chassis or the wheels are not tracking correctly. It's only slight but definitely there and not down to mirror adjustment.