Well, I hope you don't think you're being treated like an idiot, but honestly... the "scratch" (if it looks something like the example in this discussion over at photo.net) is normal for the D800/E and is not a fault.
Having previously read reports of this mark, I made a point of looking for it when I checked out a D800 and my local dealer. The mark was there, and Nikon UK had told them that they are all like that.
I also checked the D700 I had at the time and the D3s's I now have, and they all display a similar though less obvious mark under one particular angle of illumination. My wife's D600 has a faint mark too.
If it will make you more comfortable, by all means have Nikon check the mirror over - but don't count on them doing anything with it.