How to solve a miswiring of neutral and ground
The ground conductors were probably functioning as your neutral return (just not very well) until you disconnected it at the light.  So not only do you have an open (or high resistance) neutral wire, but you also have too much resistance in a ground wire.  If you've checked all the boxes and terminals for loose connections, you may have a damaged wire in the structure somewhere.  The high resistance will cause heat (and risk of fire) if you continue to use the circuit.  I would probably contact someone who has access to test equipment that can diagnose the actual in-wall wiring.  (like maybe an infrared thermometer to detect a hot spot, or a voltage detector to trace wires in the walls, or at least an ohmmeter)