I'm not sure you can be that fussy about it TBH. You accepted the display model so you cannot claim not to have known that it was not from a factory sealed box and therefore should be free of handling marks (a quick rub with a duster should clear the smudge on the LCD). As far as the power cable goes I guess it's possible than you have the box for the demo model - check that the serial number on the camera and lens matches any shown on the box or guarantee forms, if they're not the same then you have the wrong box and accessories. Of course even if they match they may have used the power cable from your camera with the demo model but even then it would be difficult to prove it...

I doubt that it will matter much if the cable has been used. As long as you register the purchase according to the guarantee requirements you are legitimately the first owner and if anything goes wrong with the camera or it's accessories during the guarantee period you simply claim against the guarantee as normal...