Just want to clarify...you received an empty package with customs tape all over it. But, it never should have went through customs. When you called customs, they told the two of you that the tracking number will show if it went to customs. The Discover CSR notated your charge back as unresolvable due to not being able to confirm if the package went to customs. Sound about right?
Customs is correct, if it went there, it will be on the tracking info as it normally sits there for a few days. You can forget about suing the CSR. The best you can do is file a complaint with a supervisor or manager if you feel it wasn't thoroughly investigated by the CSR. As for suing Discover, you'll have to contact a consumer protection attorney.
Customs won't send you anything unless there's a problem. There is a USPS holding area at customs and it should be labeled as such on tracking, but not always. You would have to have USPS confirm their distribution center on the tracking is actually located in customs for Discover. As for receiving the empty box, USPS requires something to be delivered even if there isn't anything. If all they deliver to you is the shipping label after everything else being destroyed or lost, they still consider that package delivered.
If it's in arbitration, you can't sue Discover. That's why the arbitration clause exists, to keep people from being able to sue them.
Where did you buy the item from? It's possible the seller reused an old shipping box that had customs tape on it. I've had that happen from eBay sellers. If that's the case, it sounds like the issue you have is either with the USPS for delivering an empty package or the seller for sending an empty package. If USPS delivered a damaged package, there should be a sticker or note along with it on top of them resealing the package.