First, you are not allowed to name and shame a buyer on these forums, so that part of your post will be removed.
Second, there's not a lot you can do, as it's he said, she said at that point. eBay has no proof that you shipped the item in your listing, even if you know it. There's really never anything you can do about that.
Third, the feedback isn't the worst thing, it's the fact that he claims you threatened him. If true, you're the one who should be getting in trouble. If not, you should be able to get that removed, but keep in mind eBay doesn't remove many feedbacks anymore, unless they're clearly violating expressed policy. Try the feedback removal request option, though, and see what they say (I didn't notice you saying you had done that yet, only contacted customer service, who may not have even been able to remove it depending on who you talked to).
https://www.ebay.com/sellerhelp/feedback
