Although the camera flex is modular and ''can'' be removed, the connectors certainly aren’t designed to be plugged and unplugged very often. Being not only physically damaged from the drops but also from being switched out a few times, the connector on the logic board may be damaged or have some pins bent. Take a look at it under a microscope if you can, but if another camera flex has the same problems, you may be looking at bent pins instead of a faulty camera.