Is there a way to get along with my ex?

My ex and I went through a difficult separation for an entire year, and then a divorce. We have two children together. Shortly after our separation, he moved in with his girlfriend and they have caused nothing but problems since. His girlfriend tried to have me falsely arrested, they have made false reports to DHS on me, his girlfriend forces my children to call her "mom" and her children to call him "daddy", which bothers my children because they think he won't be THEIR daddy anymore, my ex is behind two months on his child support but has bought 2 new vehicles in the past 4 months. I have not retaliated against any of these actions, and in fact, have offered to sit down and talk to both of them to try to figure something out and he refuses. They are supposed to get married in a few weeks and I am wondering if there is some way I can convince them that it is better for everyone if we can all get along.