The motor controller is a PWM (Pulse Width Modulation, see below) device. What that means is that it produces (as directed by the console) a
pulsed, variable
duty cycle signal to drive the motor,
not a steady DC voltage that rises, slowly, from 0 volts up to the max and then back down. Thus your meter, without a steady input, displays nonsense.
This process closely controls the total
current being applied to a device (a motor, or load, in this case) while maintaining a constant
voltage level for each pulse.
When you attach a battery (or any constant voltage
and current source), the full power of the source is applied to the load. Essentially, the load looks likes a dead short to the source and, as a result, the safety (fuse) blows, as designed.
So, your assessment of the problem (bad controller component) is probably correct. My suggestion of a temporary resistive load won't help. Unfortunely, it is equally possible that the console (speed selector) controller has failed. If, instead of a pulsed signal, it is sending a
steady ON signal to the MC2100, this too would cause the fuse to blow.
The problem(s) are of a sort that a multimeter
will not be sufficient for trouble shooting the system without component removal (not a good idea, under the circumstances). If you have access to an oscilloscope and know how to use one, you could observe the various inputs and outputs of the console and controller. If not, I am unsure as to what I could suggest.
One last thought: does your DVM have a "frequency" selection?
https://en.wikipedia.org/wiki/Pulse-width_modulation