I never thought about this (love this forum) but I do get the impression that when he was a young man and they first got married he was of normal mental ability, but he's not been right for a long, long time, and shellshock fits really well for that. She married a dashing young soldier, and stayed faithful to the shell of a man who came home from the war.
So sad.

I don't know the song all that well but to be honest the few times I heard it I 'read' it as a description of someone loosing grips with the world due to Alzheimer's disease.

Shellshock is a very unlikely suspect, even if you take it in its modern understanding as PTSD. At least in a Dutch context I wouldn't consider the disorder as the 'go to' explanation, even for the generation that lived through WWII. The Netherlands didn't exactly send off a lot of soldiers into situations that would make shellshock, or whatever you want to call it, a common disorder.