Just make sure you don't let her down. I can't begin to tell you the pain she'll feel if that happens.
Sadly, you've said it, you can't unring that bell, but your actions can and ought to counter it.
What's happened to you in all this, what's the background here? What was the relationship with your parents/their father with your inlaws like? Forgive me, but all your DC are seemingly crying out. No blame here, but my abuse was brought about by the lack of emotional support from my family. it also seemingly caused my depression, suicide attempt too. Went through those alone too.
Something's out of balance there somewhere. If you can find out what it is, access some help/support, you may halt all this destructive behaviour.
You do sound supportive. I'm sorry if I upset you, but you needed to see how turning your back could spell disaster. Keep talking to her, reminding her of how far she's come.
The missing him thing is common, part of the insane addiction. Good will always triumph. Remind her of how good things are without him, show her how it's best not being scared. Positive reminders incentivise. Scary threats are what HE does. She's used to them and expects it.
Report the pervert to to police for the images, see if he can be locked up.that'll help keep him away from everyone, and strengthen her support from SS.
Can she access The Freedom Programme?