I'm getting ~10,000 ohms for the two temperature sensors which is consistent with the measured temperature using an external thermometer (https://spacare.com/SensorResistanceValuesForHydroQuipandGecko.aspx)
The dry detect is for the flow sensor. The one adjacent is the thermistor and the 2 cabled one is the HL sensor.
When you say check the light, presumably you mean SPA light rather than the heater light?
The two tapped connections (arguably not very tidy looking!) are just tape around a cube connector for the cables so there is a solid connection there. It's maybe worth pointing out that I'm in europe so on 240V.
As an update, the spa has started again this morning and the HL fault has cleared. It's taken a while to get back to this point (circa 2 weeks) which makes me think there's something intermittent somewhere on the board.