In spite of the rules, I would call back and try with another rep. You've got two strikes because the 11 yr date code and the deep discount. Either one of those, but not both and you're good to go with a warranty lid.
Given the situation, that Weber got their money from the dealer years ago and you negotiated a good deal recently, I would be inclined to call back and tell them the following:
you purchased it recently, within the past year, as NOS, and name the store. Then say you don't have a receipt to provide (if it isn't too late). Offer to send photos showing its like new, but with an obvious flaw, and request a replacement. I bet you get it, and frankly, I think you deserve it more than a lot of ppl. You bought that kettle new, and it had a defect. They should make it right, regardless of the deal you negotiated.
Also, there's a chance they have a non-taco 18 sitting around. I'd ask. If not, you might check the parts companies like ereplacementparts.com, but of course you'll have to pay.