The "serial number" problem you have is probably because NikonUSA isn't that interested in supporting gray market models they don't sell. Nothing new about that. It's been a problem for NikonUSA for 10-20 years. You buy something cheap that isn't supported, why do you think you should also get free software from NikonUSA.

Possibly because Nikon made the camera and have been paid for it. The fact that you haven't used one of their favoured dealers doesn't mean they don't have responsibility for their product.