oh, my heart goes out to you.
as much as time allows, if you can build a support system i think that would be helpful. use that respite care to regularly attend al-anon meetings. the circle of friends you deserve, and should, have, need to be good and kind and understanding people - meaning they can relate to how you feel about all of this.
i agree that you need to cut this man loose, despite your fears. the fears are grounded, but you don't have a chance of serenity if you do not. let the chips fall where they may (not easy i know) and you will find solutions to the problems that come up. right now the biggest problem at hand is the addict living under your roof.
does he even help at all with the children? because in my opinion, he is doing much more harm than any good that might be happening. also realize, that you are modeled what a wife tolerates for your kids, as well as he modeling what a husband/father does.