I have no doubt that you are up against it as I am sure she is. But as judge Judy would say, he picked her and had babies with her so he has to step up and be a father!
I would like to give you some helpful advice but I will say first that I could be projecting my issues with my step mother so ignore if you wish!!
My parents had a horrific divorce and it scarred us all. It scarred my dad because while he was afraid to lose us, he didn't know how to parent us. It scarred my mum because she could only handle the rejection of divorce by lashing out at my dad. It scarred us as kids.
And when dad got someone new, it made it worse. Because we were in the midle of it. And my dad listened to the new woman who told him how unfit a parent my mother was because our clothes were shabby (he didn't pay for us) and because my sister started wetting herself when she was 13 but didn't tell people and she stunk. But my mum did what she could with what she had. She was human and made mistakes too. Dad and the new girl decided they were now better parents than my mother. It was a horrible time. It only ended when we became an age where we could see dad without her. I don't think you are entitled to form an opinion on her parenting or take credit for their improvement when you have them every other weekend.
I fully believe a child of 8 would have stress related Ibs. H have seen many stress illnesses in my work. I work with kids. The over eating, under eating, soiling, almost depressive nature of the children is common in divorce. Maybe no one is investigating the reasons because they know the reasons. A horrible divorce followed by two warring parents. And your DP is part of that too.
Lastly if it was me, and I thought my kids were mis treated or neglected I would raise hell to protect them from that. No excuses. None of this 'oh she will stop access'. That's what courts are for.
Also, as you know the best thing is an amiable relationship all sound. Hopefully one day you will achieve that. The passing of time or a new relationship can help. How long are they divorced?does she have someone new?