I suspect that much of what your dm has said to your ds came from a desire to assure him that he was doing, and did, the right thing by disclosing that your h had physically abused him.
However, now that the issue has been dealt with and your ds1 is no longer at risk from living in the same home as your h, she must be encouraged to put her reservations about future contact beween ds1 and his dsf to one side while being secure in the knowledge that she has given ds1 the confidence to disclose if it should happen again.
First and foremost, I would suggest you sit down with your dm, allow her to vent her feelings at the prospect of future contact, and assure her that, while ds1 wants to have contact with your h, you have every intention of being hyper-vigilant to any sign of wrongdoing on his part.
No doubt it's been difficult for you to square the circle between your h having physically chastised your pfb while being, presumably, a loving df to your ds2 and it may help if you can express some of your concerns to your dm with regard to the potential effect on ds2 if his df continues to be demonised by her.
After an initial meeting with your dm, it may help if you include ds1 in a further session so that he can express his thoughts and wishes to his dgm while knowing that you've got his back, so to speak.
As deep and powerful feelings have been evoked by your h's behaviour, this won't be an easy situation to resolve and I don't envy you the task of being middleman/peacemaker. However, I'm sure you and your dm will be able to reach an accord as the welfare and wellbeing of both of your boys is paramount and, as such, must override any other considerations.