A fine question, to be sure.
My best suggestion is to use "nice" not as a way of being friendly, but as a way of keeping distance. A "wall of nice," I heard it called.
She: This is what's wrong with you.
You: That's an idea. [Which it is. Just not a good one.]
She: You're such an asshole.
You: I'm so sorry you feel that way.
She: You should be sorry -- but if you really were you'd get off your lazy ass and do this taks.
You: Maybe so.
She: Do you know, some young men actually have ambition?
You: I'd imagine so.
Every statement from you is designed to a) not provoke and b) offer no invite for further conversation. My father did this with me on a visit -- I wasn't being abusive, but he shut down everything I said with "I wouldn't know anything about that."
For more on this, see Susan Elgin, The Gentle Art of Verbal Self-Defense. Also Verbal Aikido, the point of which is: don't confront; let the attack pass on by:
She: If you CARED about your appearance, you'd shave off that revolting stubble.
You: Interesting how styles vary, isn't it?
And walking out of the room after any of these gambits makes it even harder for her to sustain an argument.
Great advice on how to de-escalate potential hostile communication.
Endure it for the next 8 months.
Seriously. Trust me, you will run into many, many annoying people in your life. The upside is, right now, you have an end date. Save money, line things up, so that, in 8 months, you can move out. You cannot compel someone to like you. Sometimes, in life, you just run into people who are assholes, and no amount of nicety can change that. Toxic people will be toxic.
I've had to deal with an evil stepbitch myself for the last 20 years. She was my dad's third wife. Without going into the details, she was just horrible to my brothers, my sister (actually the step-sister from my dad's second marriage), and me.
Here's what I suggest:
1. Realize that while you might hate thoroughly hate her, and that she might thoroughly hate you, you love your dad and he loves you too. You didn't say any of that about your dad in your post, so I'm making an assumption here.
2. You and this woman are both living under your dad's roof, so you both need to respect him and do as he wishes. Again, I'm going to make an assumption: your dad probably isn't comfortable with the level of conflict between you and his current wife. So maybe you and she can dial it down for his sake.
3. I like Sir's advice for managing your interactions with her. I've had to do tons of this when dealing with my own stepbitch. Not only does it avoid escalating conflict, it will show them both some maturity. Plus your dad will appreciate that you're not flying off the handle, and actually trying to make peace.
4. Realize that while she might have a great deal of control over your father, she has zero control over you.
Couple of things. First what does your dad say to all of this? Make it very clear to her and him that she's not your mother and she WILL respect you. If they go to the my house my rules situation then I suppose you have a choice to make. I can't tell you what you should do but I know what I would do. I wouldn't be in a situation where someone could be a tyrant to me. I also know I wouldn't be anyones whipping boy. Whatever the situation is between you and her it ultimately boils down to she isn't your mother and you're a grown man. Now imo grown ups can't usually live under their parents rules after a certain point. Even if they'd like to. They could have the coolest parents in the world and still at some point in time you're going to do things that bother them, and they're going to do things that bother you. It all boils down to what you can handle. I am willing to bet she's not being nice to you. I don't see why you would lie about that. However this ultimately falls on your old mans shoulders. If he's not going to defend you then it looks like you gotta defend yourself. What you're describing is abuse. If she is verbally abusing you then you have the right/obligation to defend yourself (respectfully). If you two can't resolve issues and you're trying to do so, it sounds like somethings got to give. However I don't know the situation but I'm willing to bet she also verbally abuses your old man as well. Maybe it's time y'all stop taking it.
Get a job (if you don't already have one), join a club, get a new hobby - just be gone. Make yourself scarce by finding something constructive to do outside of the house.
I don't know how long this has gone on or how long they have been married, but since you're an adult now, you could just sit down and have an honest talk with her. Let her know you'd really like to get along for the next 8 months, and moving forward into adulthood, maybe the two of you could have a good relationship. It could work. Or not. Worth a try though.
For the sake of your father, when you are home, ignore her with all your might. To disarm her even further, do chores before you're asked. It cuts down on her reserve of "ammo" on things about you to complain/whine over. If she really is a waste of a human life and dead weight, that's your father's issue to deal with.
Tell yourself repeatedly, 8 more months and I'm outta here. Getting a job and staying gone most of the time would also help dial down the domestic tension. Remember, for you, this is a temporary situation. It's not a stopping place.