Well, I don't know enough to make any sophisticated suggestions, so I only have simple ones. Have you checked for thread right up against the motor where the shaft enters it? It can be sneaky there.
If I read this correctly, the motor works fine when the stop motion clamp screw is loosened? If that's the case, this won't help, but if not, have you tried a different foot controller?
I was going to suggest adjusting where the motor connected to loosen the belt, but it sounds like you did that.
I once inadvertantly pushed the pressure adjustment rod all the way in on a 201 and couldn't figure out why it wouldn't sew, but it sounds like you have even reached the trying to sew point, so that's not it.
Sorry, that's all I can offer. Hope you fix it without too much more frustration!