Wow Mary. What a stressful and difficult situation you are living in and I am so sorry that you are sufferring.
My aunt was in a similar situation for over 30years, but her husband finally died and and she is like a different person now. However, you can't count on their early demise, unfortunately. The only thing(s) that I can add to possibly help are probably already things that you have explored yourself, but I will toss in my two cents anyway just in case there might be something that you have overlooked.
It's easy for me to be on the outside looking in, but the way I see it is that you feel forced to stay due to the following obstacles:
1)Your disability for which you dont think that you can get an SSDI income due to lack of work credits/recent work history and can't get the SSI due to his income/assets
2)He's financially supporting your ill mother
3)You have no place to go
I understood what you posted to mean that if you could remove these obstacles, then you would leave. You have at least one thing on your side, possibly two which you can think about, and the first is that he is gone a great deal of the time. The second is that he appears to think that you are "trapped" and under his control and because of his feelings of superiority and power, he is unlikely to suspect that you would or actually could leave or get away from him for any length of time. So, this frees you up to research your options without his looking over your shoulder. In this case, you can use his bloated ego in your favor.
The most important obstacle to you is of course your ill mother's continued care/support. It might be a good idea to look into other avenues for her care such as a nursing home or assisted living facility where her social security would pay for it. If this isn't possible, perhaps your children are in a position to help their grandmother since this wouldn't be "getting in the middle" of anything between you and your husband. Also, since he likes to pretend to your children what a WONDERFUL person he is, it would be unlikely that he would revoke support of their gandmother
should you leave because that would "out" him for the *** that he has become. He might just keep on supporting her once you called his hand on it by leaving just to keep up his facade of "Mr. Wonderful". If she is still in her home and since she believes you, that right there is a place that you could go live immediately and help to take care of her at the same time.
As for SSDI and not having enough recent work history, I think that you should see a lawyer who specializes in social security disability claims because it's quite possible that if you filed retroactively from the date that your disabilty began, then you might have enough work credits in the years immediately prior to that to qualify. These types of lawyers work on a contingency basis and are bound by various laws on the percentages that they can charge. It's possible, and a lawyer could tell you for sure and probably at no cost, that you could be entitled to a lump sum "back pay" check should your claim be approved. It would be based on what you would have received from the original date of disabilty until now, in one lump sum. While this could take many months, it's worth checking into and it could
be your "ticket out".
As for having no place to go, getting social security back pay would go a LONG way in correcting that problem. Like I also mentioned, if you could go and live with your mother AND file for divorce, you could then maybe qualify for SSI if the SSDI back pay doesn't pan out. It's a no brainer that you would very very likely receive alimony or maintenance in a divorce and possibly even a monetary settlement, but you probably don't even want to open that can of worms due to the likelihood of what would be his MOST unsavory reaction. So, the more groundwork you can lay for not having to even depend on that, the better.
Lastly, someone else already mentioned for you to set aside money without his knowing it and I believe that is a swell idea. However, make absolutely certain
that you stash it somewhere where he will NOT find out about it because if he does, he will take it AND he will be savy to your plan and no doubt sabotage it in any way possible. One good way to hide money from him is with pre-paid Master/Visa cards because they wouldn't show up on any credit reports, no bills would come to the house, and there is no limit to how many you can get. A good way to get money to hide from him is to get small amounts of cash back on grocery/household purchases and sock it away somewhere he would NEVER look, like in the cleaning supply storage closet or in a vacuum cleaner bag. I used to hide money in the crisper tray because no vegetable would ever be in danger from one of my ex husbands.
Anyway, those are just a few ideas off the top of my head. I sure do hope that things work out for you Mary and I will keep my thinking cap on for other ideas.