I am surprised your correspondents on the instability of rear-wheel steering have apparently not tried the obvious!
I recently moved from Somerset, where the 100's of miles of single-track lanes regularly require reversing to pass oncoming vehicles. On occasion this might be a long reverse on an unusually straight section, where one may put the foot down a little, and I certainly find that above 10 or 15 mile/hour the steering does become unstable, and I have to brake.
Nb: No, I don't rely on wing mirrors for higher-speed reversing but turn my body in what I believe is the examiner's preferred reversing posture!
Bob Rainbow, Penmorfa, Gwynedd