I would wager the vast majority of OWs and OMs do not apologize.
We do emphasize NC, so maybe it would be best, for you, to focus on mental no contact.
Don't give this woman any more of your life. She's taken enough.
ETA: After reading further, I'd call her a "girl" rather than "woman" (and yes, I know the age of consent was past). She was not out of her teens, and had father-abandonment issues. You made her part of your family---and your pushing-40 husband used her to stroke his OWN ego. That was exploitative.
She does bear responsibility for her actions, but I think the focus needs to shift. Your husband chose to get his needs met by a troubled teenager.
THAT is a pretty big issue to explore.
I would not leap to the conclusion that she was not traumatized. If nothing else, she lost the surrogate family you created for her. Yes, indeed---she is responsible for her actions. But the real grown-up--the one who reached majority nearly 20 years prior (as compared to her not-yet-20 years on the planet)--is the one who bears responsibility for making amends to you.
Was she a stupid, fucked-up girl? No doubt.
But I'd bet her experience with your husband only fucked her up more.
I don't think she got off as scot-free as you imagine. If nothing else, she lost the family you'd created for her--and that's a pretty huge loss for a girl with FOO issues like you describe.
I am very, very sorry for your pain. Truly I am. But shift the focus. Strive for mental NC with this girl (with perhaps, first, a prayer that your husband's actions did not damage her more than she already was).
And no. I'm not saying she was blameless. I don't know whether she was or was not. I am saying that married men pushing 40 who choose OWs who are 19 and have daddy issues cause harm--and not just to their wives and children.
[This message edited by solus sto at 1:26 PM, September 8th (Sunday)]