You could always write up an explanation for anyone who needs to pick up the kids because of his "condition"--it would never be used unless he made a scene or refused to let them go. Maybe write up two versions--one for him that says "So-and-So agreed to pick up the kids if they wanted to leave, please allow them to go with her." And one for the cops (in case it ever came to that) with a FULL explanation of what is going on, as well as a number where they can reach you to confirm.
I'd kinda like to see you go with your friends. There's ALWAYS a possibility he will behave like a drunk when he has the kids. Short of supervised visitation or cutting it off, it's always a possibility. You could tell the kids in advance what the backup plan is, so they understand the drill. I don't think you should never get to make your own plans for the weekend. It isn't like this will be happening every time he takes them. It's a calculated risk, but even though the fiancee is a nut job, she's usually sober and around, isn't she?