Had old style ESA reassessment, put in support group but confused about arrears

Hello, new post from new member so I hope I've done this correctly!

After over a year of waiting for an old style ESA reassessment, I have finally been put in the support group. I was put on IB in about 1999 so went through the change to ESA but was always in the WRAG until now. I have 2 children who are now teenagers. My eldest moved out 2 years ago. My youngest is at home but left full time education this September so now he is an adult and my benefits took a massive hit because I am also on the highest daily living of PIP. I lost CB, CTC and the severe disablement part of my ESA so I am struggling to adjust to the massive drop in my benefits right now. Anyway, the letter I got says from October 2023 I would be moved to the support group. They say they owe me £639.25 after over a year of waiting, and I got a separate letter saying I owe them roughly £81 for an overpayment when my son left college (I thought i did this all well in time). Anyway, the really confusing bit in the letter says this:

"""A payment of arrears of ESA has been calculated as shown-

CB based ESA from November 2024 to November 2024 = £7521.70

IR ESA from August 2024 to 15 November 2024 = £165.45

TOTAL ARREARS £639.25

This arrears payment has not been sent or credited to you as we are holding details of a possible overpayment. We are checking whether any recovery must be made. Arrears payments due will be sent to you as soon as possible and we will tell you why any money has been held back.""""

I phoned the DWP up, the first time, I got cut off, the 2nd time they answered and said they would put me through to someone and I got cut off. The 3rd time, I spoke to a lady who seemed very nice but when I asked her what all the figures meant, she just said it was just as confusing for her as it was for me. She said I had been getting both IR and CB ESA and I may well have just got paid the one which was the highest. But why have they even mentioned CB arrears in the letter? I know years ago on IB I was getting income support. I'm just really confused. Could they owe me nearly £7500 or is this just wishful thinking? Or do I owe them £7500?? Also the lady said don't expect to get the £639.25 before christmas which I thought was really slow as it hasn't taken that long in the past to get arrears. I have already received a letter saying I owe them £81 for when my son left education, which I thought they would automatically deduct and it would be pretty straightforward but now I'm feeling that something is not quite right.

And to top it all off I got my UC migration letter this week, but that's another headache for another day. (At this stage I am just making a start at making sure I am getting all the benefits that I am entitled to before the UC change which leads me to ask another question, Could my youngest son get carers allowance for me without it affecting my benefits?

Thanks for taking the time to read this, I hope it makes sense because it's all so blimen confusing right now!