That is a smart answer he has been married to her for 20 years. It is his job he loves her. you evidently do not read the entire post and jump to conclusions (Shelly)
Personaly
All the thought out post sound good.
Add, Bi-Polar.
But if you have ruled out The change.
Just a suggestion do you think it could be or has she ever been tested for
MPD
Multiple Personality Disorder. Just a thought from my side I was married to a woman like that. May be something to look at.
But I will say that since you have been married so long wich by the way congrats on 20 years. Sit her down you have obviously learned how to talk to her and she will eventually tell you what it is that is causing her to do this
anyway Best of luck in figuring it out I personally have not figured out women yet I figure I will by the time I die lol good luck