I don't mean to be blunt but you asked for advice, then myself and other posters suggest you get a photo of the damaged iPad with the serial number showing on the screen to prove it's your device. It's one of the most common scams on eBay to buy a damaged device and claim that's the one that shipped. So that would be a simple and reasonable request which would be difficult for them to overcome if they are scamming you.
You've promptly ignored all that advice and gone ahead with what you originally said you were thinking of doing anyway. A picture of the box proves nothing.
I totally understand what you are saying. But I think my best chance is not to alert the buyer and present my case to eBay once the item is received. If the buyer is a scammer and I ask him for a photo of the serial, he can easily say the item is severely damaged and doesn't turn on anymore. Then he can sunk the ipad into water to stop it from ever turning on.
After research and calling eBay customer support, it looks like I have to accept the return regardless. I can show eBay that buyer is "trying" to return an item that is not the I sold. I still have to accept the return and wait until the item is back. That is when I can file my claim with eBay. Or they will side with buyer and refund without return.