Are your mom and brother capable of managing that? It might be time for a family meeting where you tell them they need to call him out on his shit and simply shut it down if he starts gaslighting them. If his behavior is threatening, I've had luck straight up telling my dad I'll call the cops (non-emergency line) if he doesn't stop or leave, but either your mom or brother have to be ready to commit to doing that if push comes to shove. I don't know how easy it is for you to offer them to stay with you on days your dad acts up but that can also help narcissists realize they don't have control over people when they exercise their power to walk away.

Paying his debt to keep him civil is a textbook manipulation tactic and you're not stuck in a corner. He can't hurt your mom and brother if they don't let him. At some point you guys are going to need to stop encouraging his manipulation or it won't ever stop. From experience, him moving out won't make that much of a difference.