How do I dispute an incorrect assessment on an early upgrade return (I sent FedEx, the investigation showed a USPS tracking number...)?

TL;DR: My near-perfect condition early upgrade return was assessed a grade 'C' (water damage or obvious physical damage, etc), I was charged the full remaining price of the phone (which I could have just paid off and kept) PLUS a $35 'restocking fee'. I don't believe they assessed the correct phone at the warehouse.

First phone call:

I called some time (two weeks?) after initiating an early upgrade and receiving my new phone because I didn't have a shipping label yet. I was reassured that it would come.

Xfinity store visit:

I visited a retail store with my perfect-condition phone, and they said they couldn't take it, that I'd have to work through the phone agents.

Second phone call:

After my retail visit, I called and again was reassured that I would receive a label.

Third phone call:

When I figured I was close to the return window closing, I called and was told I should have received it a long time ago, that it was sent much earlier. It was sent to the wrong email address. They sent a new FedEx label to the correct email account. I dropped the phone off the same day and received a receipt with tracking number (in August!).

Fourth phone call:

A month or two later, I saw a $500+ bill and called again. They told me the phone hadn't arrived at the warehouse, so I waited on the line as they checked with the warehouse. They told me the warehouse had found my phone and they reversed the charge.

Fifth phone call:

A month or two after that, the $500+ bill reappeared. I called again. They told me the phone had been assessed grade 'C' (water or other obvious damage, very poor condition). I said I dropped it off in perfect condition. They opened a ticket and said someone would get back to me within two days.

Sixth phone call:

A couple of weeks later or so, I didn't get a call, so I reversed the portion of the charge on my credit card. Xfinity chose not to dispute the reversal. So then some time after that, my service was cut off and I called again. They reassured me they'd handle everything, look into the ticket, re-enabled my service, etc.

Seventh and eighth phone calls:

I got an email that my ticket was closed with no explanation and a payment plan and collections notice for the outstanding charge. I texted a billing agent who looked up the information in the support cases. I asked him for the tracking number listed in the ticket, and it was a USPS number while I had sent the return via FedEx. I thanked him, called and was routed from another understanding billing agent to Assurant. That fellow seemed very intent on helping me out, but 30 mins in after a lot of silent waiting, the call disconnected. I called back in frustration, got another reassuring billing agent who passed me to another Assurant agent, who was not very fluent in English and not as understanding. He finally said he'd look into the tracking number with his manager and would call me in 30 minutes to an hour. He asked me for the phone's IMEI, which I no longer have. I'm not sure I should give it to him anyway - he should have access to the IMEI of the phone I returned.

So that's 8+ phone calls (I may have missed a few), several chats, one retail store visit, and at my hourly rate that would likely pay for a new phone. I'm beyond frustrated. Can someone from support please help me? I have the FedEx tracking number and it shows it was dropped off and who signed for it.