Menewsha Avatar Community - How to tell an abusive mother that you really don't want her meeting a "friend" to protect them from her?

Never thought I'd be in here again, but here I am.

Now, my primary partner is amazing with people. He's managed to get by in this house for months and only experienced annoyance with this woman. Never gotten into any arguments, nothing. Of course, there have been points where he would have punched her in the solar plexus if the cops wouldn't have seen her as a delicate harmless little flower and him as a big healthy strong young man who COULDN'T POSSIBLY have had ANYTHING to be physically defensive over with her, but. Point is, he's pretty much here to protect me. She's not quite as much of a jerk in the presence of someone who isn't known to be a captive audience.

If she finds out we're engaged, though...we're pretty much fucked.

We plan to move out before the beginning of January and go live with another partner of ours. She wants to meet her.

Here's where it gets ugly.

My mother, by all standards, should be in jail. She's been nothing short of a monster for as long as I can remember and has only been getting worse since she was diagnosed with lung cancer. Just a sample of the laundry list of things she's done over the course of my lifetime:

She has threatened me with scissors, and even thrown them at me for asking for them in English instead of Spanish.
The reason I was uncomfortable speaking Spanish is that I was new at it and she thought it would make learning ~~**SO FUN**~~ if she would make fun of me over every mistake I made until I cried and felt like utter crap.
When I was 9, she slapped me to the floor calling me a baby in the most venomous tone I have heard out of a human being. She said it was okay for her to do it because what I had done at school that day - defended myself from bullies - was worse.
When I was 10, she decided it was an awesome idea to throw me around by the hair, essentially BRAG about it in a "HOW DAER U REZIST?!/1!?1/1" kind of way, and threaten to kill me - because I was a bad sport at a card game, as a kid might be.
After her first surgery, I brought her dinner, exactly as she had ordered it. She changed her mind while it was being prepared and took my dinner instead. Neither my father nor I liked what she had ordered so we ended up sharing a small sandwich for dinner. The next day we heard her complaining to her friend about how inconsiderate we were for bringing her something as heavy as MY dinner.
A few days back I asked her if I could help her with something. She screamed at me. I told her that I as only trying to help, and that she didn't need to scream, she responded by telling my father to slit my throat. (By the way, it turned out that a sock was the culprit for this issue.)
She has never once apologized for any of this or any of the hundreds of similar incidents.
I'm certain I have PTSD. It's one of those rare cases where it's as easy to self-diagnose as a compound fracture. The severe flashbacks give it away. But I can't seek treatment because she's convinced that I'm the only one who's ever done anything wrong.

She thinks this is normal. That everyone has an outburst now and then. In fact, she's convinced that SHE'S the one being abused - because we can't afford to give her everything she wants. That's "controlling her" in her mind. She feels entitled to every penny that comes into my father's or my pocket. When I earn my own money, she begs me to buy her things for no reason other than that she exists. If I don't, that's abuse and I'm an inconsiderate horrible child.

She's also convinced we're conspiring to keep her off the internet, despite the fact that we leave her with two perfectly functional computers all day and have shown her about 100 times how to use them.

She believes that every time a neighbor barbecues it's a deliberate plot to kill her by smoke.

She's pretty clearly mentally ill, but she refuses help because she refuses to believe there's anything wrong with her - it's everyone else that's the problem.

Naturally I don't want my friends or partners to be exposed to this. Hell, a huge portion of the reason this partner has offered us space is so that we can BE AWAY from this.

However, I'm financially dependent on remaining on my father's good side, and he's too attached to my mother for any of our good.

My mother thinks it's a "huge red flag" if my "friend" doesn't want to meet her before I move in - when the reason NEITHER of us wants them to meet is because of THIS.

It gets even more complicated - I've lied to her. I claimed that she doesn't know this "friend" of mine, when in fact we knew each other as teenagers, took a break from our relationship for about two years because of teenager issues that we have since grown out of, then got back together and everything is awesome now.

But, my mother hated her for terrible reasons. She "just made her uncomfortable," she said.

I can identify exactly why she made her uncomfortable. One: She was a teenager. My mother pretty much hates teenagers on sight. Two: She was a guest once - she lives a few towns over - and my mother threw a fit at her for making suggestions as to what she might like to do with her limited time here when she visited. She considered this "too demanding". Three: When we were together before, I spent a lot of time visiting her. Away from my mother. She was missing her favorite target.

I want to clear this up with my father, but I'm afraid he'll take her side - even if I explain exactly why I lied. I love my father, but not when he and my mother are together. He's a horrible enabler. He disallows me from calling the cops when she is threatening me with knives or scissors - because I can defend myself and he'll stop her. Things will get better this time really~~~! he says every single time. Spoiler alert: They never get better.

How can I resolve this situation tactfully, WITHOUT getting myself cut off from the family?