Just from the brief sketch.

And sure, peoples brains are still forming, at 18, but her personality was set at 12. She knows right from wrong, even at 15 when she left.

You disengage, is what you do. Let him have the relationship they want separate from you. He cannot expect that you will forget her treatment as one poster here always states "the best predictor of future behavior is past behavior". This is true for me as well.

Due to bad behavior in the past I had to insist that SD18 Princess Powersulk Do Nada go back to her mothers and not be given the choice to stay at our house while her father is out of town for a fishing trip.

When he told me sarcasticaly that I just want her to always stay in her room anyway, I said, "no this is about the fact that she made accusations about me and threatened to call the police on me and stuck her phone in my face to record me in my home...her past behavior was bad and all I know is that if I say something that will make her mad, she will do it again, and I want peace."

He shut up pretty quickly after that.

My SD, now 62, was similar as a teen.  Whenever she was here, chaos.  She bounced back and forth between us and BM.  First, it was trouble with BM so DH was the white knight and told her to move in.  After a few months, she was in trouble here, so "mean old dad and SM" and back to BM. That went on til she launched.

Flash forward 20 years and she split with DH#2. After being evicted from 2 rentals, DH told her to move here.  More chaos plus, she lied and stole from us and robbed our neighbor.

I can't get over what she did tho DH86 has that "life goes on" mentality like yours.  So, I'm disengaged from her.  When she's here, I'm polite and civil.  But I don't call her, ask any questions, talk about her or anything else connected with her.  When she's here, I hide my purse and anything sensitive.  I dont trust her and put nothing past her.

I feel for you.  It's hard.

She's 18. 
 

Neither one of you are obligated so you have leverage to minimize your interaction with her and insist your husband do the same if her behavior interferes with your marriage. 

What does "coming around" mean? So many of our situations were or are so difficult without having an SD or BM making false reports to the police. That's a whole other level of rotten (and dangerous) right there. If it were me, I'd tell my DH that it will take time for SD to regain my trust after what she did, and in the meantime I would not stand in his way of having a relationship with his daughter, preferably outside of our shared house or at the house when I'm not there. More than likely, he'll get bitten again without you being in the line of sight. 
*Since she is now 18, I assume CS ended? My SD suddenly wanted to reconcile when CS ended. How 'bout that? Might SD need money for something now?  Is this a set up? 
 

When someone shows you who they are, believe them. 

She is not a friend of the marriage so why would you allow anyone like that around?  DH has two relationships to manage. The one with you and the one with her.  Right now the two cannot be combined.  That's not to say they could be in the future but right now they can't.  Simple as that.  SD wants him to choose.  She's happy to cause strife between the two of you.  
 

Making him visit her away from the home alone will let him experience her in her full dramatic glory. Men in general hate drama.  For now he can see her alone , if and when things change and she grows up and makes amends, you can revisit your boundaries    
 

Phrasing it like that to him gives him hope that you aren't a cold hearted bitch and we all know she won't change so you would not have much to worry about   

Make sure daddy recognizes the toxic manipulation by his low character daughter.

I would say to your DH and your SD exactly what you have said to us.  it's funny how this 18 year old girl knows what is right and wrong very well when it comes to how she is treated.  Followed by .... but treats others like crap.

Letting your DH remain unrecoverably delusional about his daughter is not the way to live your best life.

Take care of  you. Defend your marriage from the toxic failed family progeny.

Your husband can't force you to love his daughter.  I mean, I think that it's important for people to be civil and polite in a home.  You can be around her "some".. without it meaning you like her or enjoy her company.. just like a coworker or boss you may not be thrilled with.. you remain professional and nice.

I always say that I don't want to let unpleasant people rub off on me.  I may still be kind to people who may not be kind to me.. not in a 'go out of my way" sense.. but I won't look for opportunities to talk them down.. or be dismissive of them.

If your husband wants to have a relationship with his daughter. that his HIS relationship to manage.  If you can be present and pleasant.. fine.... if you can't.. find ways to be busy and otherwise occupied.

I would also say that you may never forget her past antics.. and you can be secretly guarded.. but if your husband feels his daughter has softened, matured... it's not up to you to decide that for him.. or be "offended on his behalf"... I would take interractions at current face value.. and match that energy.

I get she was not a great 15/16 year old.. would not be the first or last person at that age to butt heads with parents.  Not necessarily excusing it.. but a bio parent is not necessarily going to slam the door on their child.. when the circumstances in steplife can make relationships difficult at best sometimes... 

As far as his expectations of you "accepting her".. what exactly is he expecting... that you don't bring up past faults.. well.. why should you do that?  it isn't going to solve anything to talk to him about how terrible his child was.  Can you just be pleasant and civil... doesn't mean you have to have private girls lunches out.. or anything like that.  You can still be guarded in your feelings towards her.. but you should not make those decisions for him.. just like he shouldn't make them for you.  

I am not saying you have to agree to her moving in full time.. or giving her your personal money... but if her dad wants to see her a few times a  month.. and if she isn't actively currently doing disrespectful things to you.. then I think I would not try to stop that.

When I see a person change on a dime I always wonder what the reason is. Usually here it's either $$$$$$ or the skiddo having burned all their other bridges. 

You do not have to trust her. You do not have to like her. You do not have to cater to her. You owe her nothing. She tried to destroy your lives, he may be able to fogive and forget but YOU. DO. NOT. HAVE. TO.

Trust your gut. if you do not have cameras in home, get them. 

A back stabbing SD.  DH is crazy , he trying to play middle person trying to keep everyone happy except no one wabysxto be happy.   Tell DH you will not be disrespected.  You will NOT ascent being disrespected. You are the adult, it's your home your money , your life that you work for and demand respect.   Until SD can show respect for a year or two. You don't have to do anything. 
'DH Will keep SD out of your home. The easy place. If he wants visitation with SD. He can rent a room at motel 6 and see her there and take care of her by himself.,  see how long that lasted 

There is zero onus on you to get over the heartache she has caused and to like her again.  That is entirely on her. She has to do that work. You owe her and your DH nothing other than to enforce the standards of behavior and standards of performance you require from her and from him on all of this.

Be clear, be direct, let them both know that his shit spawn is gone until she fixes her crap. Until then, she has zero place in your life, your home, and even your marriage. Daddy can sniff her toxic ass somewhere else in a way that has zero interference in your life, marriage, and home.

You do not owe this kid or her father any effort. So do not invest your emotion and wellbing in them in any way. 

Live well. That is your duty to yourself.