OP - I think the problem is that you have limited choices, and none of them are going to resolve the situation, really.
No matter how awful, abusive or dreadful your DHs ex's behaviour is towards him and his children, no matter how disloyal you feel your DPs Mum is being, there is nothing you (I emphasis the you as I am referring to you as an individual, not you as a couple) can do to influence or change it. So you can rant about it endlessly, but no one can give you advice on how to change it, because there isn't a solution.
Your future life with your DP will always be affected, to a greater or lesser degree by the behaviour of your DPs mum and. DPs ex, How much it is affected depends on your DP and what, if anything, he is prepared to do to change the situation.
Are you willing to accept a future married to a man who spend time with his children in the company of their mum? It doesn't actually matter why he does it - whether it is his choice, her unreasonable demands, or to keep the DCs safe from an abusive father - what matters is whether you are prepared to accept that in your marriage.
If the answer is no, then don't marry him. It would be foolish to marry into a situation in the hope that it will change, when your DP has made it clear through his actions that he accepts things the way they are now.
You can't change the situation It was personal counselling that helped me come to terms with that.