Which leads to my central question: "Is there any research or specific recommendations for how to support aging BPD’s as their condition intersects with those of “normal” geriatric issues?"
Coincidentally, I've been posing the same question. If you simply "Google" the terms "geriatric care and borderline personality disorder" your search will produce some references as a starting point
If you also Google to locate the home page of "PubMed - NCBI" from that page you can run searches into the primary medical / scientific literature itself: that is basically a web search tool which specialises in medical and science literature and the universal entry point for any student / professional in those fields.
I have done some cursory searches myself and I'm not finding that much of direct relevance. So at this point I can't recommend any specific sources beyond those cited on this website itself.
I also approached informally the retired psychiatry professor of our local university, who I know socially. He could not identify any professionals in our local town who had the expertise in both geriatrics and BPD for me to seek the conventional medical wisdom. When I expanded the search through him nationally he came up with one name in a major city (here in Australia).
The unfortunate reality is that people with BPD are extremely difficult to treat let alone handle. One therapist I spoke with informally avoids such patients if she can! They are in the too hard basket.
To complicate matters, signs of dementia may be confused with symptoms of BPD in later life according to some sources.
Just some thoughts - hopefully it's OK to share them... .

My mother is 85 years old and overseas in the UK.
I have come to the conclusion that I am not suitably medical qualified to provide
any care for her (not that I want to anyway). In any case, to do so would endanger my own healing process and I have sacrificed enough over my life.
Now my mother is moving next door to her best friend who is 15 years younger: I think that's her move to entrap the BF into being a care provided (shopping, etc) should the need arise (lucky BF!). I assume the BF doesn't realise mother is uBPD an is in for a rude shock when the heightened expectations of living next door become a reality. Time will tell.
I know that sooner or later the wheels will come off that arrangement.
My strategy will be to try to locate suitable care facilities, if and when my help is sought for such or the situation has deteriorated to the point where that move is mandated.
She's on her own, with or without the support of her BF, as far as I'm concerned until such time I am, needed to sign any papers. Mother has refused to sign power of attorney papers.
It is only my personal opinion, but I would caution against the urge to get involved in the day to day aspects of your mom's care. If it's too hot for the pros to handle... well., it's a very big ask with a very high potential price to pay... .(others may disagree of course!)... .her creation of financial and other dependencies may need to be challenged or bypassed for the sake of your own family (perhaps?).
I guess it would be ideal if you and your husband had a united strategy (?). Based on my own personal experiences I must confess to having some sympathy for his position (and respect for your determination to find solutions to the inherent challenges of having borderline folk in our lives).
On a practical front, it might be worth starting the process of trying to locate any relevant medical / psych professionals with the relevant expertise you can call upon when the situation moves more into the twilight period of her life, whether or not you elect to become involved in day to day care in the meantime.
Hopefully, my thoughts and observations are not misplaced in your particular case.
Either way, lots of sympathy and best wishes
If I do find any particular literature I'll post on this thread for you.