I think its a moot point right now with questions. I think we all agree, it should have been blended, unless you had another gallon of the original paint. The mission now is to get it right. Start from scratch and go from there. I don't have Hagerty, but I hear they are very good in making customers happy. Find a place to do it right. Every paint manufacturer has their own formula for Vitamin C paint. Im sure if you have 5 brands of paint mixed for Vitamin C, you may get 5 shades of Vitamin C. Do you still have paperwork on who painted it in 2000? If you have some way to get that info, that may help. Even with the same brand of paint, it still will not be exact, but very close where blending will work perfectly.
You mentioned that the adjuster looked at the fender after it was painted? and he did not notice it was a different shade? The adjuster is not a painter, he just looks and approves money for damages. Ask the adjuster if he has a list of shops approved to do work where it is guaranteed to customer satisfaction.