BL, big hugs and support from me. It's easy for us, not knowing the person you love, the sober one, to objectively say you should do this or that, but the ties of marriage and the past are hard to overcome. Nevertheless you are clear you won't go through the extended journey to sobriety again.
You're wise to make sure everything is arranged to leave if you decide to. I suspect that's what it will take for him to understand how serious his relapse is. I know from experience that alcohol does relax us, and we begin to rely on that, but then addiction takes over and we aren't relaxed when we're away from the drink. He could have found many ways to deal with stress in his life that didn't involve the bottle, but it sounds like he didn't try.
Unfair as life is, you may have to make the hard decisions to protect the children.