Wrt a harness. If your car didn't come out with one fitted then it wouldn't have passed adr safety testing with one fitted. With a standard type seat that is designed to work in conjunction with an inertia belt if you fit a harness that doesn't have any give at all it will snap the seat in half in an accident. I wonder what that would do to your back?
I would also hazard a guess that disabling a steering wheel mounted airbag is illegal? I know you can do it for passenger bags.
I'm with MLCrisis here, it's a safety device with pros that far outweighs the cons.