It sounds like there have been many attempts. After quickly perusing the above, I have only two thoughts to offer;

1. I did not catch the details pertaining to tires. If you have not already worn yourself out swapping tires around (not pressure changes but actually moving tires) then take the tires from the right and temporarily mount them on the left and vice versa. If it then pulls the other direction, you have a tire with excess conicity (cone shape.)

2. The second thing that came to mind is something I have never seen on an MB but I suppose it's not impossible. On some of the very early integral power steering boxes on other brand cars, they would occasionally have imbalanced pressure. This means that there would not be equal hydraulic pressure in both directions while going straight down the road. On some of the early integral boxes there was even an adjustable spool valve that would let you adjust this.

This is very obscure, but it would behave as you describe. With the boxes that I saw that occur, you could raise both front wheels off the ground, start the engine and by touching the steering wheel and moving it just a touch, the steering wheel would S-L-O-W-L-Y start to turn on their own. I would be shocked if this were the case with your car, but after all you are looking for something that is obscure.

Hope this helps and sorry I can't come up with more.

Good luck,