I have the same issue on my 300SD (84-126)
I think it is the steering box on mine, because even when the box is adjusted to specs I have the play. Depending on how the box it adjusted I feel the end of the dead play zone hard or soft. I perfer the soft feel but I hate the free play. The manual indicates that one inch of play at the steeering wheel is normal. However this is the far end of normal and as you indicate annoying.
I have replaced many of the parts that you have and have been told all the other parts are OK.
I am curious as to what others say, replacing the gear box is not a cheap endevour.
Your drift is probably normal, depending on the crown of the road. When these MB's are aligned to specs this is a common event.