"...DH accepts he caused, started and escalated the row..."
Words, words. Utterly meaningless words.
Sure, he made those "I'm totes in the wrong and super sorry" mouth noises at you, like all abusers do, but look at his actions:
If he actually felt responsible for what happened, and took personal responsibility for abusing and intimidating his wife and children, then he would be thanking your mother for protecting and defending his family when he was attacking them, not punishing her. And you. And your children.
If he were truly sorry and genuinely recognised that his behaviour was completely out of line and indefensible, then he would be abjectly apologising to her for forcing her to have to do what any decent human being would - step in and protect his wife from his abusive temper tantrum and prevent him from further traumatising his children.
After all, what kind of mother would she be if she just sat there and watched her daughter and grandchildren be abused and intimidated and do nothing?
What kind of a person would just sit there and watch a woman and children being screamed at and do nothing?
What he's 'cleverly' done in the aftermath of his abusive, intimidating, utterly inexcusable behaviour is manufacture a situation that swiftly shifted your emotional/psychological focus:
- Away from holding him accountable for his disgusting behaviour
- Away from the little voice inside your head that was outraged and disgusted by his abusive behaviour and maybe began questioning his decency and suitability as a partner and parent, and,
- Towards placating him in order to retain your relationship with your mother and the respite and emotional support and connection which she offers.
...Successfully, as evidenced by the fact that you're actually placating him and practically begging him to 'forgive' her and permit contact with her for you and your children instead of looking at him with rightful disgust and disillusionment and telling him to get over himself and snap out of it lest you lose further respect and love for him.
What he's also managed with this handy little manouver of his is next time, you'll think twice about disagreeing with him/objecting to his abuse, and your mother will probably ignore all of her instincts and think twice about protecting you, as she'll see staying silent as the lesser evil when the alternative is you and your children being completely cut off from her support.
....This right here is why abusers just don't change.
There's no introspection, no taking personal responsibility, no bearing of the emotional and psychological 'weight' of their own bad behaviour.
Another person might glean life lessons and personal growth from the aftermath or consequences of their own actions.
In an abuser, accountability, responsibility and shame are all either deflected or weaponised in service of protecting their own ego and interests, which is always, always their first priority.
"...but says as it happened in our house DM should apologise to him..."
Riiiiiiight.
Just so we're clear:
A man abusing and intimidating his wife and children is wrong, but not if he does it in his own house.
Stepping in when you witness a man abusing and intimidating his wife and children is right, but only if he is abusing his wife and children in a public place, not in their own home.
Once the king of the castle steps within the threshold of his front door, he may abuse whomever he wishes and no-one should do anything.
So basically every domestic abuser ever is in the clear, and anyone who has ever called the police because the king of next door's castle was tantruming like a man-child, or because women or children were screaming or crying, is actually wrong and should apologise. The police who showed up and (maybe) arrested him should apologise.