Thanks everyone for chiming in, it’s good to have multiple viewpoints.
Nordost replied to me and according to them the difference would not be audible, though since the surface was compromised the copper will start to oxidize but it might take many years. Ultimately the level of performance degradation, however slight, would put it somewhere between Frey 2 and Heimdall 2.
I’m leaning towards making this a keeper and push out the thoughts of “oh it could be even better”, but yes the resale value will no doubt take a hit. I might not care about that if I use it for the next 10+ years or more, just have to get over the psychological part!
As for the seller, it appears to be a honest mistake and oversight and we are sorting out a partial refund.