Hi @chrisbayne
You specifically mentioned NPD in the title of your post. NPD is one of the four personality disorders (PDs) in the cluster B category of the 10 PDs in the current version of the DSM, DSM-5-TR (2022).

It’s possible for an individual to be pathological, difficult to deal with when they have enough traits but not an official diagnosis of any specific PDs. NPD is particularly evasive of official clinical diagnosis some of the defining traits work against the individual to make it into a diagnostic setting.

Mayo’s description & context of PDs is, IMO, well written:
https://www.mayoclinic.org/diseases-conditions/personality-disorders/symptoms-causes/syc-20354463 . One of the key points I keep seeing from many credible sources is that the problematic traits need to be pervasive/emduring.

While psych stuff isn’t my “native” area of knowledge, it would seem to me that dementia(s) & other neurological conditions happening on top of existing PDs would manifest in specific/distinct ways as each of the conditions can affect the others in how it plays out.

Some PDs in some individuals might increase or increase in intensity over time/with age. My personal reading of the medical literature on NPD in older age is either increase or decrease can happen, depending on other factors. But then add dementia & symptoms can +/- .

Best wishes for peace for us all!