Hello! That part in the photo is a adhesive release strip. Nothing to do with your problem. But on the other hand if the phone will talk to the computer but doesn't show anything on the screen. Means there is something wrong with the screen. Have you had it replacement recently or dropped it?
Hello! Not sure which camera your talking about here. If the ribbon broke on the camera facing towards you it will affect your ear speaker and proximity sensor. If it is on the one not facing you then it does not affect anything but the camera it's self.
Hello! You certainly tried all the things I would have. It seems like your phone is having trouble trying to pick which cell tower to talk to. Doesn't seem to be a defective antenna but most likely the logic board. Which is not repairable or worth replacing.