You are married to a man with, frankly, a mother who sounds insane and grossly over posessive. However you have one big advantage. She lives a long way away.
Your DH clearly understands the problem he has tried to talk to his mother and has tried to set boundaries. What you now need to do is to enforce those boundaries.
So start by blocking her number on all your phones. Then your DH rings his mother once a week at a regular time and tells her all the news. He could send her the occasional email, but if she sends abusive emails, block those as well. If she abuses you to him, he simply puts the phone down and doesn't ring until the following week. He needs to make it absolutely clear to her that you are his wife and he will not tolerate any criticism of you.
As to visits. Agree between you how much you can tolerate of her presence. Frequency - 2/3 times a year, once a month, whatever you choose. Then whether his mother is in your house or you are staying with her, your DH has again to have zero tolerance of any criticism of you. If she starts he goes silent, doesn't respond and/or walks out of the room. He should allow himself, say half a day with his mother without you being there, but after that insist that any outings, events etc must include you and the child.
Once both of you make a determined and inflexible decision that you will always police the boundaries that his mother will not obey and that every one of her transgressions will end in her being cut out and ignored. She may, after a couple of years get the hang of it.
But the thing to be clear about is that this woman is out to break your marriage, by any means fair or foul, and the two of you have to present a united fron to deal with it.