It sounds to me like you haven't really forgiven your DP for his behaviour, you've just glossed over it. If he was truly sorry about his aggression towards you he would have worked out what was going on with himself, done some work on his behaviour and knowing how unacceptable it is, wouldn't now be tolerating the same from his DD. If he was really on board with helping her he wouldn't need you to encourage him to get his DD counselling, he'd be doing everything he could to get her (& himself) some help.
You say "I'm willing for us to make a huge effort with SD" but actually I don't think this is your battle. Your SD is witnessing the same behaviour in her other home from her sister and has probably seen it from her dad in the past, despite what you think, so any influence you have will be minimal. She's witnessing this behaviour all around her.
I think I'd be moving out and letting DP decide how he wants to bring up his DD without leaving myself my DS in the firing line.
"I fear for my son, and for my safety."
That's no way for you to live and it's unfair on your DS as he can't make that decision for himself. It's your job to protect him, not by getting between him and SD while she's kicking off, but by removing him from the situation in the first place.
FWIW, my DP has had anger issues in the past. He has always been hot-headed, not violent to me, but aggressive, throwing things etc. I couldn't put up with it. It came to a head one night and I was scared he might totally lose control and hurt me so I called the police and finished with him. I was not going to tolerate that sort of behaviour.
He has since read a mind management book which has really helped him, apologised to everyone involved, including my DCs and family and completely changed the way he reacts to stressful situations. We've had a couple of counselling sessions, which weren't particularly enlightening, but acted as a starting point for our own discussions.
Because his outbursts had always been tolerated and indulged with a sort of amused 'oh here he goes again' acceptance by his family and ex, he has never really seen it as a big deal or felt like he had to do anything about it. Fortunately his relationship with me is important enough for him to sit up and take notice of my expectations and boundaries and finally learn to address things in a different way.