That poor poor child.
It is clear that he cannot come to live with you if you are not allowed to guide or discipline him and it will be demanded that you allow him to hit you and the baby and to swear at you. I realise he is only 6, but is it reasonable that you should allow that?
So no, I would say no if that was the deal.
I would say that he can certainly come to live with you, but he must be guided and disciplined and he will be expected to behave appropriately and he will face consequences for unacceptable behaviour.
That boy is crying out for rules, for boundaries - for safety! He must feel so unsafe. He needs consistancy in his life. He needs guidance.
You could change this child's life.
But if you are expected to take him into your home and you already know that any attempt to modify unacceptable behaviours will be met by world war 3, then I think you should say no. and in your shoes, I would be saying that perhaps husband should move into his parents house to care for his son.
re SSM - it sounds to me like she is out of her depth and quite possibly seriously bullied by your husband's parents. They've never given her the chance to be a mother and she possibly now feels she's not up to it. Or maybe she's never had to grow up and be a mother because they took the baby much of the time. Or perhaps she's not the maternal type. Who knows what the deal is there?
What matters is that there is a little boy who is being failed. He needs love, consistency, boundaries, rules, support. He needs a home and a family.
A normal family!
He's 6. It's not too late. He's just a baby. Imagine if nothing changes - what do you think he will grow into?
I wish I could take him myself. I really do. I honestly and truly do. poor, poor boy.
And I feel for you to. I get the feeling you would be more than happy to try with this boy, and have a place for him in your family and heart - if you were not expected to allow him to behave in totally unacceptable ways.