If you have a problem with a seller and can't resolve it then you need to turn it in. (You did)
The seller would not resolve even after a dispute had been started. (Unusual)
eBay would not send you a refund until you sent the part back and provided tracking and the tracking showed it as delivered.
You didn't want to send it back (Understandable) but eBay will not refund..
The only work around would be to send the seller a box of the same size and weight with tracking.
What the box contained would be up to your conscience
Of course the seller could report and it may delay your payment but it seems to me that it would be your word against his and when that is the case, eBay typically sides with the buyer.
Regarding the email you received, I believe you only have 45 days to change the feedback so it's way too late unless he could get a court order. (Fat chance)
Further, if the email was sent through the eBay messaging system, look at the bottom of the email.
You should be able to report the email as harassment.
I checked his feedback, what a idiot.
If we have a problem we take care of it even if it's not our fault.
I've got one I'm working on right now, and it's not our fault....
No warning, no dispute...
NEGATIVE.
I got his phone number from eBay and called him immediately.
He was really mad but after I calmed him down and figured out what was wrong it turns out that he had a different problem entirely.
He has apologized and said he would change the feedback but he hasn't yet so we'll see...