Unfortunately it sounds as though you got through to the village idiot when you contacted eBay the first time with regards to getting the feedback removed. Try contacting eBay Customer Services again in the morning using the option to have somebody call you back, as that's when you've got the best chance of receivinn a call from an eBay representative who actually speaks English as his or her first language, thus removing the problem of trying to get around a language barrier and make your case clearly understood by the person you are speaking to. Lines open at 8:00am and close at 10:00pm.
In order to contact eBay Customer Services click on the Policies link at the bottom of this page. When the Policies page loads up scroll right down to the bottom of the page and look at the left hand side of the screen. There is a mobile phone icon there next to the words "Have Us Call You." Click on that link and an eBay representative should contact you shortly afterwards. Have the item listiing number to hand, as if you give this to the eBay representative when he or she calls you this will help him or her to locate the case faster.
Once the eBay representative has pulled up the case details argue your case strongly for the removal of the buyer's feedback. Make a point of referring eBay to every one of the messages where the buyer threatened to leave you negative feedback and point out that this is feedback extortion and is in breach of eBay's feedback policies. Once you have made that point say something along the lines of "On the basis of the clear evidence of feedback extortion in relation to this matter, I would appreciate it if you would remove the buyer's feedback from my feedback history." Once you have rammed that point home it will be somewhat difficult for eBay to refuse to remove the buyer's feedback when you have clearly pointed out to them that the buyer has breached eBay feedback policy. Therefore, if you are lucky enough to get through to an eBay representative with an ounce of sense then he or she ought to overturn the original decision not to remove the negative feedback, in which case it should be removed from your account. If this does indeed happen then the feedback-related defect that your account would have been hit with as a result of receiving that negaitive feedback will also be removed, in which case this buyer's feedback comment will not have done your account any lasting damage.
Good luck with getting the feedback removed, and if you haven't already done so make sure that you add the buyer's User ID to your Blocked Bidders List by clicking on this link https://offer.ebay.co.uk/ws/ebayISAPI.dll?bidderblocklogin. A big white box will appear on the screen - add the buyer's User ID to that white box, click on the Submit button beneath it and once you have done that the buyer who left you that negative feedback will never again be able to purchase anything else that you list for sale on the site.
