Electrical problems are so tricky. Seems like you are tracking the problems down quite well though.
Logically a switch is just a switch. If you see no evidence of circuitry around or in the switch then you can eliminate that.
Could the switch have worn enough to send current into the return wire?? I think you said that you tried 4 switches on this regulator and it only works on the drivers side.
I think your switches are OK
So how is the resistance to motion? The motor, or circuit controlling the motor, shuts off when it reaches a certain current level. Could the mechanism be so stiff to operate, that it is fooling the motor into thinking it has reached the end of it's travel??? does the window go up and down easily without the motor attached???
This is all just guess work - perhaps it will help???
Rich