First of all, you won't get in trouble with the police for sending her and angry letter, providing you don't threaten to harm her in any way or word in in a way that could be construed as blackmail. That doesn't necessarily mean you should send it though.
Sorry to nitpick and it's not that I don't believe you but I am really confused about some of your points.
He's had to move out of the family home (a mutual agreement whilst he gets better, we continue to function as a family)
Where has he gone? Not back to his mother's I hope?
She is a very high earner, works from home, controls all the money so FIL DP and BIL maybe get £10 per week.
If your DP is a mid to late 20s professional, why is he needing to have money controlled and rationed by his mother?
And his brother? Does he not have a job?
He is mid (to late) 20s and has never even had his own mobile phone (lived on his own but she's that bad, He's only got one this year)
Again, how on earth has a grown man of professional standing who lives with his partner and his children, allowed his mother to control whether or not he has access to a mobile phone all these years? 
She stopped him from coming to the hospital or tried to anyway, she told him he had to stay at her house after the assault and banned him (mid to late 20s, professionals) from using the internet to connect with his family saying she turned it off to save money and from using the phone at all saying what if someone needed to call me? She wouldn't let him visit his newborn daughter and would get incredibly violent if he did
Why on earth was he allowing this to happen to him? How can she demand that he stay there instead of in his own home? And where was your son at this point, while you were in hospital and your DP was being bossed around at home by mummy? She's infantalising him in the most awful way - and he's letting her. Was he already very ill at this point? What is her issue with you? It sounds like she thinks you are really bad news for some reason and that in her own warped way she thinks she's trying to protect him from you.
He called her when he was really desperate asking if he could borrow some money and explaining he had tried to kill himself a few times and she said no, that she would love to but his dad wouldn't be happy about it (obviously his dad was out at the time she would never say that if he was there), She told him if he wanted any money at all he would have to go back and live with them and she wanted to meet his psychologist so the psychologist could persuade him to go back.
After all she's done, why would he call her and ask her, instead of speaking to his dad and seeing if his dad could persuade her? Why is he even asking at all?