Rob
Good to hear your parents emerged relatively unscathed from such a severe accident
If you're not into car stripping yourself it would be worth talking to the site sponsors
http://www.a8parts.co.uk/ to see what they'd offer for the rest of the car.
It probably depends on the severity of the accident, but given the insurance company have allowed you to buy the car back it won't be registered at the most severe level which would require distruction of the remainder of the car.
Given the level of damage I suspect the useful parts are going to be limited to small bits of trim that didn't get broken or damaged in the accident and parts at the rear of the car.
I'm not aware of a fuel cutoff (there's nothing in my manual for my S8 that refers to one). Given the severity of the accident I'd be very surprised if the block wasn't damaged - if it won't fire it may simply be due to wiring damage around the front of the engine bay.
Regards and best wishes to your parents for a speedy recovery
Adrian