I designed a steer by wire system back in 1984. It was for a skid steer loader. The diagnostic software could pinpoint most failures and correct the situation. The only thing that was beyond control was a failure of the final power transistors on the drive to the positioning motors. We could diagnose it in 7 thousandth of a second, but due to capacitance, could not guarantee that the loader would not go into a hard turn. Now, the idea of a backup clutch on the manual link is a great idea, as long as it is exercised on each startup to check for proper operation. I love computer control systems because nothing can go wrong, go wrong, go wrong........oops!