Hi There nothing much you can do if Apple think your iPhone already been tampered. Your only option is to get Machine to confirm when you send in your iPhone to Machines its verified and was not open/repair before(because they are the one that have to open up your iPhone to verify before they can send it to Apple). Only Machines can prove or provide that verification. If Machines refuse to do anything then you are out of luck.
You can rant about it in social media and create a big fuss and hope either one will succumb to help you.