I do sympathize with the plight you are in. I also understand all of the reasons you listed...but for me (and only speaking for me here) the unknown is the scariest of all places. Right now it's very difficult for you to grasp the possibility of actually being happy if you leave, or ask your W to leave...whichever.
Failure....is led by pride. I'm oh-so familiar with that one too. I can certainly tell you that one of the reasons I am working soooooo very hard at this M is because I don't want to feel I've failed.....again (oh and I do love my H). But you know what? It's not failure if you try everything humanly possible, and you my friend have. You have given a heroic effort with the cards stacked against you. I'm not telling you what to do here, just pointing out you haven't failed at this M, you have worked harder at it than most people will ever work at anything. You haven't failed this M.
Now as for family, history etc. Why do you believe you'd be cut off from an entire half of the family? Do you truly believe they would cut you off? This doesn't always happen, and it doesn't have to happen if you have good relationships with these family members....especially for your girls sakes. Some of that, is up to each person.
Unfortunately sometimes we come to a crossroads in our lives where we are forced to choose between two unpleasant options, sometimes there's just not a "good" solution. I was once faced with one I thought I'd never get over (and I was harder on myself than anyone ever could be.) It involved a Bi-Polar xbf of mine that I was trying to disentangle myself from. He threatened to harm my family and I was forced to deal with him on my own. I was faced with staying where my family was (and facing the possible physical harm of one of them, because he was very unpredictable and quite serious about it) or coming back and facing him one on one....therefore ensuring no one else would be hurt. I knew if I came back to face him my father wouldn't understand, and he didn't (afterall he couldn't be there to protect his little girl)....in fact he didn't speak to me for over a year (I won't go into details). So I was faced with hurting my family on my own and knowing it would mean very hurt feelings and the possibility that my mom or dad would NEVER speak to me again...but hoping they'd eventually get past it, or facing the possibility of my xbf truly harming them physically to get back at me for leaving him (this was in my younger days).
I finally took the road on my own, and prayed my family would one day understand why I did what I did. Know what? They did eventually understand....we eventually became much closer for the whole ordeal as well after several years, and it made me a better person. But no one, and I mean no one....was harder on me for what I had to do to my family than I was. The hardest thing I EVER had to do was forgive myself for hurting my family, in order to protect them.
ZB, the way I see it...that is similar to the choice you have. If you leave...you will be protecting your girls, but at the same time you feel you will be hurting them and your W, and one day, maybe much sooner than you even realize....your daughters will understand, they love you ZB...and you love them that is soooo apparant. The thing that makes this sooo very difficult is you love your W too.
This time you have to choose what's in the best interest of you and your girls...either way it's not pleasant, but try not to be too hard on yourself.....ok?
I don't know that anything I had to say here helps...just trying to be supportive of you.
