Mel,
If it were me, I'd get off the 'cause' kick. It does nothing more than cause (no pun intended) anxiety and stress, which is not good for PN- that's proven!
Do what makes you (Alan) feel better and live with it.
Get on with your lives and try to live as normal a life as possible.
Concentrate on the things you can try to do something about (healing the foot).
Believe me, once the 'acceptance' mode kicks in, you'll both be better off.
Never be complacent, always be aware of new and better ways to diagnose, but stop going ga-ga nutso about finding the cause. Your posts seem to be highly detailed, and anxiety ridden- whether its you or Alan. Let it lie.
Just MHO.