Especially GMs at high end dealerships (overall a generalization but I have met more idiots and/or scumbags than not). They think they are a providing a specialized service. No, I'm here because the car sells itself - you just happen to be the middle-man.
OP - "Office Bully" is paydirt for HR and I like some of the strategies of baiting her in front of cameras or responding to texts "I don't understand, what do you mean?" Screenshots and get it via email are clutch (email especially because IT has that all backed up). Definitely keep taking the high road and kill her with kindness. If she lashes at you in the break room (or whatever) Say something like, it sounds like you're having a bad day, anything you want to talk about? What can I do to help?
Problem is (and has been stated) if she is eye-candy at a luxo shop, she is fighting from an elevated position and they will let her toxicity slide until they start losing sales because of it. Pretty sure she knows that and can game it; she seems well practiced.
The Art of War is a book I recommend everyone read. One of my favorite quotes is: "If you wait by the river long enough, you will see the body of your enemy float by". Meaning, don't engage them (her) on their turf and let them self-destruct. Have patience.
Oh, and as a back-up polish your resume and start shopping yourself around. High end shops are a small community as you probably know and if you have a clientele list or contacts, you will get snapped up.
Keep your nose clean and relish in the drama. It's stressful, but fun to push someone to self-implosion. Psy-ops is an artform.