Thanks Dinged Heart, I've got some experience in this under my belt.
And as the more ethical out of me and my ex, I hoped for the best but prepared for the worst. I can't stress enough how glad I am that I did. When my son's father did seem to clean up after it was all settled, I relaxed about visitations from what was court mandated. But when he went back to the same destructive patterns, I had the legal ability to keep my son from harm during that time. As well, because my ex is so petty when he is on a destructive spiral, I had it arranged so that all visitation travel is his responsibility. I knew that he would use it to jerk me around with scheduling and location arrangements if he had the ability.
If Sun ends up getting his head together afterwards, Carma can always conduct things ethically in light of it. But it would probably be best with so much at stake to not count on that or believe it will be a permanent change if he does. Seek at least a custody specific contingency plan that will work if he just gets worse or vengeful.