There are a lot of tricky relationships here and I believe you are focused too closely.
I would recommend looking at this system by viewing the EHA current. Although eventually you will see the same sort of closed loop evaluation for making adjustments, but tthe EHA current is what really matters, so viewing it is the window to seeing whats going on.
In other words if you thought that some form of constant or imtermittant signal was taking place with the coolant temp sensor then you would see the results of this in the EHA current.
The other thing about the EHA current is that it is real easy to achieve the default limp home state of zero milliamps; just unplug the EHA.
The system works properly when the mixture is set to 0.5 - 1.0% CO by exhaust gas analyzer and then the electrical correction is engaged. If the mixture is such the system will read it from the O2 sensor and keep lambda control through closed loop correction above and below 0.0 milliamps EHA current.
This is why one can adjust the system electronically without an exhaust gas analyser. but if things aren't in closed loop control the EHA current will tell more about whats wrong than the dutycycle at the diagnostic port.
Bosch Master, ASE Master, L1
33 years MB technician