The 100% reading (96) indicates the car is too lean. The fixed reading is either a result of that or the airflow pot as described.
While the reading is 40% there isn't much you can adjust, but when it is 100% try pushing down on the airflow meter flap. This should richen the mixture and if the O2 sensor is functioning the duty cycle should drop before you push so hard it starts running bad.
You should also disconnect the O2 sensor while reading 100%, it should immediately drop to 50%. I think the O2 sensor code 50% is the prevaling code. So it might even go to 50% when it is 40% if you disconnect.
Your explanation of grounding the O2 through your body is wrong! The test goes like this: simulate lean by grounding the sensor the duty cycle should head high. Simulate a rich mixture, run battery voltage through your body to the computor lead from the sensor: the duty cycle should go low. Running the O2 sensor input to ground through your body probably won't do anything.
The EHA current should go to +12ma when shorting the O2 input to ground and the current should go -12ma when the term sees battery voltage through ones body. (DO NOT HOOK battery voltage straight to the sensor input - we are trying to simulate 1-2v)
I have changed the sign convention twice now after rereading this. I think I got it right.
Bosch Master, ASE Master, L1
33 years MB technician