I agree with you more every post you write!
Blocks of cardboard wouldn't fade if they were used either as they wouldn't generate enough friction to do so.
I'm not sure I follow the harness discussion completely. I fitted my harnesses using eyelets screwed into the outside original seatbelt mount and a reinforcing plate on the tunnel, with the straps through the holes in the seat - that's the only way to do it, surely, as you don't want them mounted to the seat or the subframe? My shoulder straps are fitted to eyes where the rear lap belts mounted, but I considered that that was too low so I never used the harnesses. I understood the angle you're looking for is +/- 15 degrees of horizontal and the 45 degrees is a long way in excess of that. My intention was to have a full cage with a bar to attach the shoulder straps to both for optimum angle from the shoulder and to shorten the length of the shoulder straps. I also planned to brace the seat back to it.
I had decided on a full cage after much deliberation finally reasoning that I have the seats as low as possible, my cage was to be a custom weld-in one with no bolting plates at the A or B-pillars and I'd use good padding above the door, which is still not ideal, but that would allow me the best rigidity and allow me to add a door bar which I would expect to add safety. My understanding is that the only negative of a full cage on a road car is the possibility of head injury due to hitting the bar over the door top without a helmet, but have I missed something?
]) Donington isn't the hardest on brakes, particularly in a slow car with road tyres, but I've also seen them used elsewhere quite successfully. 
