I don't see any reason you can't install a damper on your W123 turbo. You should be able to buy a generic "throttle damper" someplace and make it work.

Regarding 300D turbo manual transmissions, I think the reason is simpler. AFAIK the turbos were only sold in the US market, and I do not believe MB was selling any manual transmission cars in the US by 82. As I recall, even the SLs were all autos by then. I think that was just the "package" they chose to sell in the US. If they wanted to sell turbo W123s with manual transmissions, they would have made it work.