People tend to seek out relationships with those who they're familiar with. Your husband being emotionally unavailable, is why his daughter had found a similar partner.
I can see why your husband is against the boyfriend. In no other area of life would someone be able to "threaten suicide, threaten to cause damage and have police involvement" and still be expected to just get over it. But in relationships it's minimised and acceptable. Your step daughter's boyfriend behaviour have been minimised and justified because he was young and stressed. Yet her dad is saying hold up why are we acting like what he done is ok. Yet the daughter wants everyone to move past it for the sake of the relationship.
Speaking from experience I was glad when some of my family would get over the toxic abuse incidents that I would tell them. On one hand it made my life easier. But on the other it had a very negative and profound effect as it meant that I didn't seek out their support anymore. But that was worse as it meant the isolation escalated. But I had family who were not prepared for me to be isolated so they kept the peace for my sake. But that was actually worse for us, as it meant this demon was escalating the emotional abuse instead and seeing that he would be "forgiven" and could get away with it. In retrospective I'm glad I had a few family members who refused to minimise it as it kept me focused and not allowing the abuse to be downplayed. When you're in that cycle and fog you don't think logically, you live in this weird bubble which is what I suspect your step daughter is doing. As tbh as it's been pointed out already by PPs the cycle has already begun. Her feelings about her dad and his past behaviour are valid, but they should not be her sole focus, so she can just minimise abuse that she has dealt with in her relationship. I used to do that, focus on others behaviours, as it was the one thing I felt at the time that I was able to control.
I would suggest you stay in regular contact make the effort to meet at the park etc and explain to your husband that she is at risk of being abused and you can't afford to isolate her further. If you make a plan to have regular contact perhaps away from the home, it means when her partner at some point starts to isolate her by niggling in her ear not to visit her family as her dad doesn't like him, you'll be able to pick up the pattern and nip it in the bud.
Sorry for the long post, but I can see how this will play out and I wished it could be stopped before it becomes her normal. xx