w/o/w: I am familiar with the situation, but unfortunately do not know anyone who came back after leaving for reasons of verbal bullying. I am only offering my knowledge gained from being closely involved for a period of time, and because you seem to be looking for this type of help. I do not know what will be the right approach for your family, I am just offering suggestions you may not have heard before. I would certainly defer to any expert who has more knowledge than I on this particular subject.
Shabbos/Yom Tov is one of the hardest struggles. Are you able to maintain an atmosphere of rest and peace at all for the sake of the rest of the family? I think certain rules must be followed in “public” areas at home, but I don’t know if you can win the battle of what goes on behind his closed door. You might have to set out a list of the melachas that you do not want to see in your home on Shabbos, so that you can keep some semblance of Shabbos decorum. But it might be a losing battle to try to enforce shomer shabbos at all times in all places for him. The Shabbos/Yom Tov struggle makes the biggest case for living on their own, and yet, when they go out on their own, they may abandon any vestige of self-control with respect to Shabbos restrictions. Do you have a family Rav that you speak to about Shabbos?