As I have my bike on the stand I did not notice it was, in fact, variable power. Just not variable speed as when not under load. The bike is not ready to ride. I am still testing...

As it is now, when I advance the thumb throttle a little, the motor does not engage. As I slowly advance it further, the motor eventually engages. I back off a little and the motor disengages. So that would be the "low threshold."

So I advance the throttle slowly until I get engagement and then I advance the throttle further and further but the motor never moves any faster through the rest of the throttle's range of motion. This behavior is effectively "on/off" and there are no speed differences anywhere on the throttle range once the motor engages. Are you saying that I have to load some software or get into the configuration interface in order to add more sensitivity or incremental detail to the way it reads the throttle?

I'm not sure I understand. What is this provision in the wiring harness you speak of? I see that the control cable has three pins. Are you saying under a different configuration, this could be a real throttle? Or are you saying somewhere withing the wiring harness are wires for a real throttle but are not exposed to the user? What I have is the "thumb throttle" but it has no sensitivity and is effectively just "on/off." I suppose I could disassemble it to see if there is any sort of potentiometer.

The thumb throttle has either a potentiometer or a hall sensor.

I think your thumb throttle could be defective. Or your settings to the controller could be wrong. Try another, higher level using the + button.

It is the tester which was defective. I failed to take into account that I was testing on the bike stand and with no load, variations in speed/power are harder to detect. That said, there is no "near-zero" speed on the stand which I sort of expected. Either way I think I have my expectations calibrated now.