I want to thank you for this thread, its made me learn more than I thought I ever wanted to know about my IP. It seems the thread goals have "jumped" around. ie, What if the solenoid failed? How can we start it with a failed solenoid? How can we shut it off? Can we use a valve? etc...
The PDF supplied was enlightening and taught me how the Fuel Shut Off works. I surmised a manual system to override the solenoid in case of a failure prior to the PDF being posted. Turns out, according to the PDF, some actually have a manual override to allow emergency operation in the event of a solenoid failure. The concept is a good one though very unlikely it will ever be needed in our personal HMMWV's.
The thought of shutting down the engine using a fuel line valve has too many flaws IMO to be a workable solution, as others have pointed out. Realistically the shut down process of releasing the solenoid to stop the engine is all but fool proof. Literally turn off the power and the engine stops. There are very few if any component failures that would prevent that from happening. That said, it has always been my limited understanding of Diesel engines theory of operation, the fuel is metered but the air isn't. The engine is allowed all the air it wants at anytime but the fuel is changed to control the engines RPM's. If you stop the air the engine also stops. It would seem the best method to emergency stop the engine would be to introduce an air block off of some type. A large butterfly valve similar to a wood stove dampener that is cable operated would be pretty easy to do. However, the relative ease of killing the power to the solenoid with a switch would be just as effective IMO.
My conclusion for my vehicle is the current design needs nothing. I guess if my world revolved around my HMMWV running then I would consider an emergency override to allow my engine to run in the rare instance the solenoid failed.
