I have early-set through declining MCI, and my wife is a narcissist. This is going to be very interesting when I slide into dementia (it's coming, the signs abound) and her narcissism goes into full bore.

I don't have a coping mechanism for you, unfortunately. If you want a bigger audience - much, much, bigger, then maybe go to Reddit and search for the sub "Dementia". You'll have 1000s of like-minded people in this group - I believe there are 34,000 members - which you can join, and people are posting several stories just like yours every day. You may get some helpful suggestions out of it.