Oh StrongEnuf, ...if I could reach into my screen, across the miles, and hug you, I would.
We are very similar in many ways. My ex-MIL also told me to leave her son. In fact, her intervention was part of what helped me leave. I told myself that if a man's own mother is warning women against her son, then there's something drastically wrong with him. As weird as it might seem to some, I am still very close to my ex-MIL. She has just gotten a computer and I now email her daily, send her pictures and videos of my daughter.
It seems as though what's holding you back now is your own fear of the unknown.Even your inlaws are behind you and know that you DESERVE BETTER than your AH. They see him for what he is: a parasitic leech who will destroy his children's lives if given the opportunity.
You are lucky to have their support in this. A lot of SR members have to deal with enabling and accusing in-laws who make the separation and divorce process a lot harder.
As for your fear FOR him, let me say this: you met the man when he was 28. During his teenage years and adulthood, he didn't die because you weren't around. He did just fine, like another adult. Yes, he's learned to rely on you and use you in the past 15 years, but given a change of circumstance, he will figure things out for himself again. If he wants a place to stay and food, he can find shelter and a recovery program, free of charge, at the Salvation Army. There's help to be found out there, if you ask for it and go get it. Finally, he is not your responsibility. Shrug off that mantle because it's not yours to carry.
As for all those WHYS, simply put, you weren't ready. You hadn't reached your bottom. No sense in torturing yourself more than he already does.
I firmly believe that everything happens for a reason...and that your Higher Power (or God, or Buddha, or whatever you want to call it/him/her) is now giving you the opportunity to change your life for the better. It's time to focus on yourself and stop thinking about him.