I'm not sure I really understand the point of this thread having read it back.

Manufacturer makes car, fits seats having crash tested them and the car to meet the required standards.

Man buys car, fits harnesses that the seat was not designed to accept.

Man crashes car, seat fails.

That's like dropping a 10,000 tonne weight on the roof of the car and then complaining that the roof collapsed.

The seat has been loaded in a manner which it was never designed for. Not really surprising it failed is it? Don't see that Ford or Sparco can possibly be blamed in any way.