Electrical – Should Ground and Either Neutral or Hot be Lighting Circuit Testing

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)

The grounds should be bonded to the neutrals at the main panel. (If it's not, it's a serious problem, call an electrician.) If you are getting good grounds on some of the outlets, it's probably bonded at the main panel.

If the ground isn't spliced right* at a receptacle it will show no-good at that receptacle and at all the receptacles on the same circuit downstream from that receptacle. That would be a possible explanation. It fits with the miswiring by the previous owner. Tracking down the bad ones can be a bear. Still that may be better than tracking down and replacing a damaged wire.

*edit: by "spliced right" I mean all the ground conductors of all the cables and all the ground terminals of the devices are bonded together in the box, and the box itself it's a metal box ...

If there was some creative wiring going on, some wiring can cause the receptacle tester to show a good ground where there isn't. For example jumpering the neutral to the ground terminal on the receptacle (non-code and a very bad idea) would show a good ground on the tester, but there isn't.

If you can double check with a different tester, that might be a good idea, you don't want to chase your tail if the tester is no good.