Having read this I don't think this is necessarily a winder problem, it's possibly a shutter problem that can be traced back to ageing electronics or possibly to the cold weather outside or to insufficient battery power. I think you were right to return it though.
I had something similar happen where the shutter wasn't closing in extreme cold (-15c) and the camera was on a tripod for long periods of time. I think this was down to insufficient battery power to close the shutter. In those temperatures the film advance would also freeze up after 20min or so sitting on the tripod. After a few hours inside the camera continued working properly.
I think I've mentioned this before on this forum but a respected technician once told me after servicing my "6" that there was nothing particularly frail about the wind mechanism, the real problem seems to be that certain parts can be hard to find if the mechanism does get damaged. I guess I was lucky that in two years of using mine I didn't have any film advance issues apart from freezing the camera.
I would still look for a good 6 over a 7, the collapsing lens binacle and the built in finder with frames for 50, 75 and 150 make it a much more convenient camera than the 7.