iPhone 6 backlight issue, burn-in
The iPhone warranty in the US doesn't cover physical damage or cosmetic issues. The scratch will likely be judged that way unless it interferes with a function like multi touch.

Similarly, the return policy / window might be your best bet - especially if the LCD quality is clearly different than the rest of the display phones. Just because a light scratch happened - if there's a problem with the overall display, I would assume you'd be able to request an exchange / repair / warranty service to address the backlight and not the scratch.

If Apple's repair option fixes the scratch too, so be it - that's their issue and not yours IMO.

Lastly, get in sooner rather than later. Also, if you didn't buy the phone outright - be sure your carrier is OK issuing a second subsidy before returning the phone.

Sorry this isn't simple - but you've got multiple aspects to a repair/return to consider.