You're not the one on the wrong. Well, apart from for staying with this asshole.
Dont raise your child on a household where they see their mother abused for a second longer. You're financially independent? Great! Get out there!
Move as far away as possible. Make it difficult for any of them to spend time with the kid. Fuck.them.
Yes they'll paint you out to be the bad guy, but who gives a diddley as you'll never have to spend any time with them again. Conduct any context messages through your ex partner (or Heck, a solicitor) and block the rest of them from your life.
Of course there is the worry of unsupervised time spent by your child with them. That sucks yes. But here's the thing, my gran was an abusive cow. She made my life a misery irregardless of whether or not my mum was there. She could put you down in a room full of ppl and no one would be any the wiser...but you knew it was a dig at you.
But what really hurt, was when she used my mum against me. And also, when she treated my mum badly. When it was just me, I eventually learned that she was q horrible old boot and what she thought of me - why give a shit!
If you create a safe home environment away from that family, if you keep yourself safe and if you are always supportive of your child when they come home then that will be far better for her/him. Because then at least there is restbite. And one person in the world they know always has their back. So that if the other side of the family do treat them badly, they know they will never be alone if they find the strength to yell them to sod off. Just like their awesome mum did.
I wish mine had found that strength.
I forgive her and I understand.
But please get yourself out of there. Your child should gave at least one safe home. Qnd q happy, brave mum to look up to. Not a martyr and a fool who stays and suffers and cannot protect anyone, not herself or her child. Get out.