GFCI receptacle works partially when switch is turned off; not at all when on
You have confirmed my suspicion that, with the switch off, the outlet and the light are in series. That is the reason that both light dim when a 100 watt light is plugged in.

The fact that the outlet shows hot/neutral reversed when the switch is off tells me exactly what the "electrician" did. The outlet must be wired as follows:


the hot side of the outlet (small prong) must be connected to the white wire between the lamp and the switch.
the neutral side of the outlet (large prong) must be connected to some hot wire (probably black).


This way, when the switch is on, it is shorting out the outlet (hot-to-hot). When the switch is off, you have a full path from hot into the neutral side of the outlet, from the hot side of the outlet to the hot side of the lamp, and from the neutral side of the lamp to neutral.

Bottom line - the switch and lamp seem to be wired correctly. The outlet is miswired. Frankly, the wiring of the outlet is so screwed up, I wouldn't let the same "electrician" back in your house. Find a real pro to straighten it out.