So it looks like electronic control works. You should have gotten negative current flow in the 9.9ma direction.
This range of control will usually make the engine run poorly in one direction or the other. Remember that when you ground the O2 input the system thinks the engine is lean and it richens up the mix by the 13ma (in your case).
The next step is to set the basic (uncorrected) mixture to .5 to 1.0% CO. This is hard to do on MBs because there is no precat test port.
Since the system seems to respond to created O2 signals, one might suspect the real signal, but lets test it. Hook your volt meter to the O2 sensor lead (leave it disconnected from the controller). If you get 1v the system is rich (or the sensor is lieing). Use the 12v thru Cap'n method to lean it out. Does the method make the engine run poorer? Does the voltage drop below 1v?
Bosch Master, ASE Master, L1
33 years MB technician