M., sending you big hugs. My situation is similar to yours, except that my kids are either neuro-typical or only have mild LDs and manageable ADHD, so I don't have the added stress of young kids needing services. I can only imagine how stressful this is for you.
My husband just moved out over the weekend. Like yours, mine has a long history of mental health issues but fortunately, he has always been able to work (albeit in a job where he vastly under-earns what someone with his education and experience should earn). Like you, we have done years of counseling.
In preparing for this, I read this great book:
http://www.amazon.com/The-Truth-About-Children-Divorce/dp...
Even with divorce not on the table right now as your primary goal, it has good info about preparing yourself and your children logistically and emotionally for this transition. It gives very practical advice and describes the predictable phases that you and he will go through. There will be some variation due to his mental health issues, your depression and your kids' needs but it is helpful to understand that ambivalence, anger, guilt, etc. are normal and to be expected.
My husband's initial reaction to my request that he move out was that he would when his name was off the deed of the house and until I was able to re-fi him off the mortgage (impossible because we are upside-down), he wasn't going anywhere. From a legal perspective, I had no recourse. You can't force someone out of his own home. So if he owns your home with you, be prepared for the fact that he might refuse to go and unless you're willing to leave, there may be nothing you can do about it.
What got him to change his mind was hearing our kids describe our home in therapy. They all expressed unhappiness, described our home as tense and angry, identified him specifically as the cause of tension and the one who needed to change, and my step-daughter actually moved to her friend's house this summer and refused to come home. Hearing this prompted my husband to act and make steps to move out. Honestly I'm financially screwed now and will probably lose my home, but I can tell you that right now, it's nice to not have his miserable black cloud in my house all the time.
If you haven't done so already, ask divorced friends for recommendations on attorneys and make appointments with a few of them. Most will do a free consultation. Even if the object is reunification, it's good to know what the law is for your case in terms of temporary support, whether or not your state recognizes legal separation (mine doesn't), etc. If he doesn't work, he should have some source of income from disability and should be able to use that to support himself and pay child support to you. Otherwise, you're going to have to figure out how he's supposed to pay for a separate place and contribute to your family's finances. Being disabled doesn't relieve him of his obligation to provide support to his children, and your eligibility for services for your kids (childcare, transportation to therapies) might be dependent on his participation in your family's finances (i.e. they would expect him to pay something first before any additional benefits to your kids would kick in). You may also need to show that you're on the path to divorce before you qualify for additional benefits for your children. It's a complex legal and social services area and I don't pretend to understand all the ins and outs, but that's something you should look into.
If I were you, I'd try to get answers to some of your legal questions before you tell him. If you can't, at least line up some appointments and let him know during this conversation that you will be seeking legal advice to ensure social security will be available and know what you need to file for a legal separation (if anything) so that he understands how serious you are.
Best wishes to you, and take care of yourself as you go through this!