I believe that by manually shifting the trans you will be stressing it more than normal shifting. I don't see it as any worse than any other form or hard driving and the car is designed to take it. It also would probably be wore out if it was anything but a MB at this age.
I don't believe that shifting manually is necessarily the hardest driving, it just facilitates it.
Bosch Master, ASE Master, L1
33 years MB technician