I'm writing to ask for perspective & advice on aging care for a toxic parent.

The red flags are waving in your face!!

You've waiting 6 years while he cared for his GRANDFATHER, and now he is proposing moving his abusive mother in?

Run.

Concerned; Forget the diagnoses. She has a personality disorder.

He is her slave and unless HE wants to change his behavior, there is no hope for him to have a full life outside of his relationship with her.

If you haven't already seen in, watch the movie Moonstruck. You will see what happens when folks are the ones waiting for the mama/daddy whomever to die.

It never ends.

You guys put your lives on hold so you could care for his grandfather, her dad? Now you are expected to place a further hold so you can care for her because she doesn't want to live alone?

He has a choice to make and it appears the he has already made it if he hasn't married you after all of these years. It sounds like you are free labor for him to take care of his dysfunctional family. Do you really want to procreate with someone that puts everyone above you?

I would pack my bags and run from this situation. She is using her past to manipulate her son and thereby perpetuating the abuse in the family.

Once she is in the door, no matter the feelings of either of you, it will be almost impossible to get her back out the door. If you feel strongly about this it is time you BOTH, your husband and you both, attend counseling. This could easily break your famikly before it starts. Your husband is now married. The first imperative is now to his own family. If his mother needs placement for safety then this should be done. Toxic parents create children who are their slaves. They work a lifetime to do this. Without clear boundaries, which in part will be set by YOU for YOUR life, this will spiral out of control. He is not STUCK, your husband. He has an oath of loyalty to YOU and your future children. You do not say how old MIL is, but she could, I assume, live for decades.
You are going to have to take charge of your own life if your husband is unable. For myself I would make it clear that my relationship with him could not continue with his mother in my home, and I would leave that home, and the MIL in his care if he insisted. I would say this gently and with love, but I would make it clear that I am not willing to sacrifice my own life for his mother.

You can only change your behaviour, not his, not his mother’s.

It appears the apron strings were never cut. It is highly unlikely they will be at this late date.

So your options are to live with your fellow and his mother or move on from this relationship.

If he could look at the situation unemotionally, he might be able to see other options. Independent or assisted living, where she will not be alone.

She could volunteer to get a sense of purpose, engage in a hobby.

But unless either of them decides to change he is going to either move in with his mother or move her in with him.

Many people who have grown up with dysfunctional parents, toxic, personality disorders, etc. have a very difficult time facing reality. It's difficult to really confront the issue, because, it's all you've ever known. Most of us develop skills to survive and denial is common. Rarely, can an adult child just break free. It's complicated. I'm not sure that an ultimatum is inappropriate, but, regardless of what he decides, he may waver later, if mom continues begging for the attention. Only with significant counseling, would I consider it. I would examine factors, such as: Your age and how long you have to actually plan a family; the resources to care for the mother; what is mom's life expectancy, is she very ill and may pass away soon? Lots of things to consider. I'd just be ready to accept that toxic people, can make your life miserable, especially, if they are living with you. I wish you all the best.

There are a number of posters who are or have been in your situation. Lately one even moved out before she posted.

I will assume your in your mid 30s? So Mom is not quite 60? If so, she is not that old. Diabetes can be controlled. The person doesn't need care 24/7 for it. Carpal tunnel can be corrected. Does not mean she is disabled. Have you looked up the meaning of Narcissist? Does she fit the discription? Passive-Passive-aggressive? Does your partner realize that by moving in with his Mom his life will not be his own. He will be at her beck and call 24/7. This is not a normal way to live.

Right now Mom has lost a partner. She needs to adjust to her new norm. She needs to do for herself. Your partner can help but he should not be her whole world. He is her son not her husband. Your partner needs to set boundries. What he is willing to do and not do. Mom could be part of your lives but you should be his #1 married or not. You too will need to set boundries with partner and Mom. This forum has recommended a book called "Boundries" by Townsend and Cloud.

What your partner may need to learn is he cannot make his mother happy. She needs to do this for herself. If she suffers from depression, she needs to see the appropriate doctor. She needs to find things to do to keep busy. Maybe hard to clean her house, but she needs to do it. Its all up to her.

You do not give much information on his mother as to her health status other than abuse. (I do not mean to trivialize this when I say "other than")
She is asking him for help maybe because she knows he is a safe person to ask.
Therapy is a good idea. But if he starts to care for her YOU should also be included in this therapy.
I certainly would not start a family with this looming at the horizon. And if you have put this "on hold" for 6 years already how long can you or are you willing to wait?