I don’t believe that your analysis is completely correct.
https://wiki.openstreetmap.org/wiki/Key:motorcar clearly encompasses (both in legal practice and in OSM) all relevant “dual tracked” vehicles.
You are correct in saying there is not a simple mapping of OSM access tags to legal restrictions and the other way around. But law isn’t speciality of math and the motorcar quirk is just far more practical than any alternative mapping.
Extended, your suggestion would require us to re-map motorcar=… to motor_vehicle=… motorcycle=yes mofa=yes which I don’t think would past muster anywhere.
Simon
PS: note none of the above is really CH specific.