The disability insurance route doesn't seem attractive to me -- it takes a while to get approved for the money and you are usually not allowed to work at all while you are collecting. I am not sure if you are talking about short term disability or SSDI, but both can be very limiting in what you are allowed to do.
Are you in touch with SCORE or any other business development agencies in your state? A business plan that includes a profitability forecast and a cashflow analysis would be incredibly useful for your situation, to let you see what you need to spend to make your business bring in enough income for you to quit your dayjob.
Would you be able to find a different day job, maybe a part time one, to supplement your income while you devote more time to your business? If you removed stress from your marriage (by leaving), could that make the job more tolerable, or vice versa?
What would your back up plan be if your business does not generate profit before you ran out of money? Would you be able to quickly find another job to cover your bills?
Is there anything your current employer could do to make you stay? If you are at the point where you are ready to quit anyway, it doesn't hurt to ask for more money or better hours or whatever.
Will you have to pay any taxes or penalties if you cash out your retirement savings? I was bummed when I cashed out an HSA and had to pay 40% in taxes!
Sorry, these are a lot of questions
For the past four years or so I have been doing what you are talking about doing -- spending what should be my retirement savings on my sanity instead. I wish I had left my first misery/insanity inducing job much sooner than I did!
There's a lot to be said for how much better it feels not to work somewhere you hate. At the same time, not being able to pay the bills can also be stressful. I am confident that I can get a job that will cover basic living expenses as soon as I need to based on my skills and location. If I were in an area where jobs were harder to come by, I might not take this risk.