Could anyone advise on Amazon's policy in the following situation?

I have a customer who returned a completely different product, likely by mistake. I followed all of Amazon's guidance available on the help page and emailed the buyer three times, but I received no response.

I opened a case with Seller Support to seek advice on how to proceed, but they simply provided a copy-paste of the Amazon help page (which advised reaching out to the buyer), or to open a Safe-T claim (which is not applicable since it wasn't refunded by amazon) without offering any further assistance.

In the end, I used the Manage Return tool to charge a restocking fee. I noticed that by selecting "Materially different," Amazon allowed us to charge a 100% restocking fee, so that's what I did. I also emailed the customer once more, asking them to reply to my emails.

However, shortly after that, the buyer opened an A-Z claim claiming that he did not receive a refund. Amazon closed the claim in the buyer's favor, refunded him, and counted a defect against me. I appealed the decision, but there have been no results thus far. (I did not receive any notice of why my appeal wasn't accepted.)

I would like to know what to do in cases where a buyer returns a different product, and it wasn't auto-refunded by Amazon (therefore not eligible for a Safe-T claim), and the buyer is not responding to emails. How should I proceed with a refund? And if I need to refund them, why is there an option to charge a 100% restocking fee? and why it's counted as a defect as Amazon does not provide any guidance on this?

So far, I have not found any guidance from Amazon on this matter. My experience has been negative, as the buyer successfully defrauded me with an A-Z claim.