Complaint sub-issue: Credit card company isn't resolving a dispute about a purchase on your statement

This complaint concerns my Discover 1 credit card and an unauthorized charge of {$250.00} made to a CLOSED credit card account on XX/XX/2021, and then, without informing me, was transferred to my current open Discover account. I immediately contacted Discover to dispute the charge, and they re-edited my account and supposedly stated an investigation. However, when they contacted the billing agency, they were told that the agent indeed had the CLOSED account information and the administrator mis-informed Discover that they had requested permission for use of that account for recurring charges, which was untrue. But, Discover restored the original unauthorized charge to my account balance and added interest for the period of time from the initial complaint to the next months bill. When I contacted them to dispute their action, Discover told me that I had to obtain a letter from the billing agency admitting to the unauthorized nature of the charge and they would then remove the re-instated charge and associated interest. I obtained such a letter ( attachment # 1 ) from the billing agent, who admitted that she had recently taken over billing for the association and had just assumed that authorization for recurrent billing had been obtained by the organization. Discover refused to accept a digital copy of the letter admitting the error from the Billing agency, so I had to mail it via the USPS to their Delaware offices, even though Discovers headquarters are in suburban XXXX. Since I heard nothing in response to the documentation requested, I called regularly and after many hours of being transferred to people who claimed they were not in the proper Department, many hang-ups, and hours waiting to be transferred, all the while they kept the charge and added additional interest each month during the period they were supposedly investigating. 
Finally, after more than 60 days after they received the first letter documentation, the person I finally reached who would tell me something, told me that since the biller hadnt indicated the $ amount of the unauthorized charge, my documentation was incomplete and no investigation had been done by them. Totally frustrated, I recontacted the billing agency, where the woman told me that the monies had been withdrawn by Discover in XXXX, and she provided me now with a more detailed letter about the refund to Discover and a copy of a letter from the charge processing service also stating that the monies had been refunded to Discover in mid-XXXX. By now, it is early XXXX, and Ive been charged increasing interest on the disputed charge on at least 3 monthly bills ( I continued to pay Discover the full authorized balance due each month ). These letters ( attachment 2 ) were mailed to Discover on XX/XX/2021, and I was promised a call back within 5-8 working days, but never received any communication. So, I started calling Discover again, each time asking for a Billings Dispute Supervisor, as no one else would help me. After 5 calls totaling 10-12 additional hours of waiting, dropped transfers and wrong Department connections, I finally got a Supervisor today XXXX XXXX ) who put me on hold for 30 minutes to review my complaint # XXXX ( apparently for the 1st time ), and she finally admitted that my last set of documentation was complete and adequate, and she would credit me back the original unauthorized amount of {$250.00}, but she refused to credit me any of the $ XXXX in interest charged over the 5 month period on this disputed amount. When I asked to speak to someone above her who could authorize the interest refund, she hung up on me. Then, in this afternoons USPS mail, I received a letter from Discover ( attachment 3 ), dated XX/XX/XXXX, stating that Discover had reviewed my additional documentation and had rejected it, recharged my account for the full unauthorized amount plus interest and reported that decision to all four credit report bureaus!