I can understand your anger and frustration op. But you need to channel your energies into getting the best outcome for yourself and your child rather than fighting a system you will not win. I went through all of this with my grandson, who lives with me. I spent too much time, energy and stress trying to right CMS.
My gs's father tried every trick in the book to pay nothing for the son he professed to love and care about. The reality was he earned £60,000 and claimed to earn £39,000 self employed. From bitter experience I knew unless I actually had his wage slip in my hand, I could not prove it to the CMS standard. I accepted the £177 pm and then he did not pay that for 12 months.
Each time CMS took enforcement action, he would make one payment which meant I was back to square one because he had "complied". They did take him to court for a Liability Order which meant he legally owed arrears. He had to pay the court costs. Because payment was made via CMS, he had to pay something like 20% on top. I think he might have owed them as much as he owed us.
They even looked at taking his car so he hid it at a relatives, they looked at taking out a charge over his house so he transferred it into his parents' names. This was over £177 per month.
Long story short, after years of issues it went to FIU, their fraud dept. Tbf, the caseworker did his best and his final shot was telling him that he did not accept his accountant's paperwork and asked him for 4 years wage slips. I wanted a lump sum settlement because I knew he would stop paying again. Surprise, surprise he paid up rather than hand over wage slips. I closed the case.
Don't be me op. You cannot polish the turd that is CMS. That said, without them my gs would have received nothing. Focus on getting what you can. I am not criticising you, just offering you my advice via bitter experience.