But if Apple believes that original proof of purchase (the receipt that the author presented to Apple in hopes that it would convince them to unlock the laptop)

But if Apple believes that original proof of purchase (the receipt that the author presented to Apple in hopes that it would convince them to unlock the laptop) is insufficient (and let's assume the lack of cryptographic signing of said receipt isn't the problem; Apple can verify the receipt another way) because the original owner could've sold the laptop to a second owner and then stolen it back, then why would Apple think it's ok to accept "your private key" as proof that you're still the owner? You could've sold the laptop and stolen it back -- the exact hypothetical situation that prevents them from considering the original bill of sale -- and then used the key that you had possession (or knowledge) of the whole time! A secondhand buyer should demand the key if it's physical, but if it can be duplicated this breaks down.