I'll join up for your posse any day, little lady. Can't say in all honesty whether I bring much sanity into the equation, however.
I think your instincts are, as always, spot on. Your husband wants to hurt OW when what would be best for him is to just let it go. He should ignore her, and stay out of it with her. She obviously "got" to him, and he's reacting unwisely -- that's not a good sign.
It also tells me that your H still has a long ways to go.
I think back to those people who hurt me in my life, and if I saw them today I would want nothing to do with them. Life is too short to waste any energy on them. (My CW is now joining their ranks, BTW.)
You're doing wonderfully, Lwb. My hugs and prayers are with you today.