Waaay back, I asked about reconciling the resistance issue because it is THE step we need to address in order to confirm if the problem is in the circuit or in the motor.
NOW that we go that out of the way, we can safely conclude that the problem is within that motor. It runs fast so the motor mechanicals and brushes, etc. seems all in good working order. The problem is in the internal speed control circuit board. Once you get that back cover open, you will likely see a small PCB (I bet it's the same one with extension that protrudes from the side where the wires connect to). There will be a handful of components on it and a IC. Since the service manual does not provide the internal circuitry schematic (owing to that these were usually replaced as an assembly when they were available), your best bet is to find the IC and check the datasheet on it. The datasheet usually includes a sample circuit diagram which will likely match up quite closely to what you have in there. You already know how to do component testing so if the issue is not in the individual components, it's likely the IC. An example of some are: AN6650 /51 /52.
On most cassette motors, the back cover is held on by several crimps. However, the method differs depending upon the motor. If you want to split the motor bad enough, I'm sure you can do it. Good luck.