Stuck in rut

OK so I need help and opinions. I'm going to start this story from the start so that you can get a good grip on what is going on.

I met my partner almost 2 years ago, we dated for a bit and then soon fell into a relationship. He has two children from a previous marriage and I have 3 from my previous 10 year relationship. When first going into this relationship it was awesome, I hadn't experienced those nervous butterflies when first meeting someone in a long long time so it was all new and exciting again. After being together for a few months we introduced our children to one another, and then he began staying at my place with his children on his weekends with them. It was hard work and a lot of patience needed going from 3 to 5 children. I found me and my partner worked good together along side the children, we definitely had or ups and downs, mainly when there was conflict between kids and we were all still learning how to handle this new blended family situation. We moved in together after 6 months and had his children with us every weekend. Everything was going pretty smoothly in our relationship except one thing......His ex! Now I must say me and my ex get along very well and always try our best at co-parenting for the sake of or children. We never push and pull them. Now this woman I had to meet was all kinds of evil, she would try to hold the kids from my partner, play games with the children, say horrible things about me and my children to there kids. She would move around alot, changing the kids schools constantly all for the benefit of what was the flavor of the month at the time, his children were always very unsettled. This was sad for me to watch as my children always had stability and routine. All my partner could do was try and communicate with his ex about what was going on, most of the time he would just get replys telling him to F off. She would manipulate my partner and try to control him, and if he didn't do what she said she would hold his kids from him. This frustrated the hell out of me! She started causing conflict in our relationship. She screwed with my partners mind so much. I found out that after we had been together for 6 months he had had a sexual relationship with her, she couldn't have been more happy to let me know about it. When I found out about this we broke up for a while, I felt betrayed and used. After a lot of yelling and crying we did reconcile. I tried and am still trying my hardest to forgive him for everything. So anyway we moved on and moved back in together and lived semi smoothly for almost a year, even tho she was still lingering in the background trying to stir crap we tried to get on with our lives and do the best we could with raising our children.

When we would pick his children up for there weekend with us they would come to us smelly and unkept, they would tell us she had not bathed them all week, nor had they been to school for the week because she couldn't be bothered taking them. Some forms of physical abuse was also happening at the mother's home. My partner had had enough so we rung the police and cyfs . the children were then interviewed and placed into our care full time. And now begibs the custody battle😱... She tried everything to make out my partner was a bad father, she's even so sick and twisted and tried saying he would sexually abuse all the children. The courts seen straight thru her little tactics. We were granted the day to day care of the children. So any way now I get to the main reason behind this post.... She has requested with the courts stating she would like to do counseling one on one with my partner so they can get along better and I'm not OK with this! If you knew this woman the way I did and how evil she really was you would understand. I know her games and manipulation tactics. I still have major trust issues with my partner and I know she will do and say anything to get her own way. Should I stay with my partner if he does this counseling or am I setting myself up for disaster ? Help