In my opinion there are really two separate things here. Practically I think the only sensible thing to do is to take good advice, and then get a divorce as soon as you can. I would also in the meantime make a list of all you can remember, that was spent on your mil, so was not available for you and your husband. It can be quite surprising when you see the complete list over time and realize how it has impacted on your life. By getting a divorce hopefully you will at least a voice being responsible for his or her debts.
In the meantime I suggest what I often suggest to people in difficult situations. So you could take two pieces of paper. On one you write all the positive things there are about your marriage, what you enjoy sharing, what you admire in him etc. Each time you think of something write iton the paper, and then fold it over so that you cannot see it. On the other piece of paper put the negative things about your situation. Then on a day you feel quite calm and relaxed and on your own, open these papers and then what you are looking for is not a list but to look at what you have written, and see how various things group together and it helps you to see what is important to you and what you might not have realized how important an area is to you. You may actually be surprised by somethings. While you are in your situation, you are not taking a step back and this can be helpful.
I think you must still have some love for your husband as you have remained with him during this time. So having looked at your lists, thought about your situation as it is, that should be the time when you decide what you want to happen.
So, it may be that you see that you need to divorce, and move away and have nothing more to do with them. This would be sad for you, but at the same time allow you to have a clean break, and get on with your own life, without the constant worry about the debts etc. Or you may divorce and either ask your husband to leave or leave yourself and get something smaller, perhaps a flat or something that you would be able to cope with well yourself. Put it is your name only, but then you could still invite your husband to spend time with you, but on your grounds. Make sure that he knows that you will not mortgage your flat or give him any money for his mothers debts, but you could still invite him for meals and perhaps to stay with you occasionally. That way you keep contact, but on your terms, and you will feel less stressed, knowing you are safe yourself.
Only you can know which if any of these ideas might work for you. Do you have a close friend who knows your circumstances and might be able to help you to look at what alternatives there are? The main point of these ideas are to show you some possibilties for you to take control of your own life. You will never change your mil's attitude, nor does it seem likely that you can get your husband to alter his attitude, or if he did , he would probably feel so guilty about it that it would still not work for you both. So the point is that in reality we cannot change other peoples behaviours or attitude, only our own. Perhaps this seems like a new way of thinking about your situation, so I would urge you to think about it a little without involving anyone else. Then you could come back to the post and if it appeals you could begin to write those lists and see where it leads you. Hopefully to a less frustrating and worrying way of life. Wishing you strength to do whatever you feel able to do.