The relays in N82 are changeover types so depending on the direction of movement, either terminal of the motor can be live.
Have a look at the diagram here -
Change-Over - Products - Narva
It is a repesentation of half
of one of the relays in the N82 unit.
For the purpose of this explanation, the terminal '30' is connected to one terminal of the motor. Pin '87' is +12v and pin '87a' is connected to earth. Pins '85' & '86' are the coil - ignore them. Within the same physical relay is another one of those. On this one, pin '30' is connected to the other terminal of the motor but '87' & '87a' are reversed i.e. pin '87a' is +12v and '87' earth. ( It may help to draw it on paper ).
In the state illustrated then, the one half of the relay has pin 30 connected to earth and the other half is at 12v. When the relay is energised ( both halves change state ), that swaps over and pin '30' of the first half is now connected to +12v and the on second half pin '30' is now connected to earth.
It's actually a bit more involved than that as in that case the motor would never be off. But it allows you to see why you cannot rely on detecting +12v at the motor.
The microswitch is irrelevant in respect of normal operation. It only influences the autodrop feature and the microswitch only provides an earth signal on the sensing pins ( right door & left door ) of the N82 and merely signals to the N82 that the window has fallen sufficiently for autodrop. For the purposes of your fault finding, you need to move the adjustment cam as far away from the microswitch as possible. Once you've moved the cam away from the microswitch, you can ignore it altogether.
If there was a problem with the earth, the locks, door open warning light and puddle light would not work. The earth in the door is not relevant to the window anyway as both wires of the motor are wired directly to the N82 unit.
As the window is free moving, I'll give my thoughts on the electrical side.
First thing to do, is disconnect and reconnect the connector between the door and 'A' pillar. A high resistance owing to corrosion there could be influencing the controller. Disconnecting and reconnecting it should be enough to clean the contacts.
If that doesn't fix it, I'd then try and isolate the controller and drive the motor directly. With the window halfway down, disconnect the motor from the door wiring. Get some good thickness wires ( capable of carrying at least 10 Amps ) and connect the motor to +12v and earth. KEEP FINGERS ETC OUT OF THE DOOR, YOU DON'T WANT THEM SLICED OFF BY THE REGULATOR. The window will go up or down. Switch over the polarity to make the window go the other way. You're looking for the motor being capable of moving the motor over the full travel of the window. You need to be ready to disconnect +12v when the window gets to the top and bottom.
If that test is successful, I'd then suspect the current limiter in the N82. I'll detail that later depending on your other tests.
Hope that helps.