I would not totally give up on this at this stage. Whilst it may be a longshot,if you have a photo of the coin that was submitted to NGC and it had no corrosion on it then and came back with it , then on the face of it , this will have happened whilst they had it and they owe you a duty of care in handling it. You , or if it was submitted through a dealer, the submitter should I suggest write to the CEO of NGC in a measured and unhostile tone, expressing your/their extreme disappointment at what has happened , that you have used them a lot satisfactorily in the past (if that is the case)and saying that you expect to be reimbursed for the damage that has been caused to the coin whilst it was with them and that they have not complied with their duty of care whilst it was with them.
They will obviously resist and blame anyone else they can and assuming that they respond negatively to your first approach , I would suggest following this up with a threat to draw attention to what has occurred with the coin media, which you might eventually want to follow through on if they do not play ball.
As I say this is a longshot but for the sake of a few letters/emails it may be worth trying.
In terms of what they might eventually offer you , if at all, I guess that it they will not give you the resale value but might give you what you paid for it.
You could also at the same time contact the RM as a customer of their's and ask them what they think might have caused the issue given that they have metallurgists working for them.
A friendly dealer eg @ Lawrence Chard , might have a better suggestion!
Good luck