Start taking steps to separate yourself from your toxic mother. Get together the paperwork and the funds to get your name and your name only on your car, so all your dad has to do is sign it. Give her back anything other than clothing that she's given you, and stop accepting things from her. Call up your relatives yourself and tell them, "Hey, just so you know, I'm going to be taking an extended break from dealing with Mom. Please don't communicate with her about me, and don't bother trying to pass along any messages from her." Then change your number, and, if possible, move. Give your new information to one person you trust to keep you up to date with family issues without sharing that they're still in touch with you with your mother. Make a new Facebook, refriend your own friends, but not any family members that are friends with Mom, and delete your old one. If she and EvilGranny tend to make fraudulent phone calls to the police, go in and meet with the police and give them a heads up.
Get some counseling - even just joining a support group for people with toxic parents could be helpful, and is typically free.
You say you have a good relationship with your dad, but I can't say I think much of a person who doesn't protect his or her children, even against the actions of their other parent.
Honestly, this isn't really an etiquette issue. You're dealing with an unreasonable person who isn't operating on normal polite/impolite standards, and you won't be able to deal with your mother on that basis.