I get VERY hot, I have to put on the sunscreen several times a day, and the black leather seats BURN when you get back in, even if only parked for a few minutes to get a cold drink at a servo.
Actually I find it helps to carry a couple of white towels in the compartment behind the seats, throw 'em on the seats when leaving the car topless. It's another clear blue sky day today, probably heading for only about 28 degrees though as there's a fresh breeze.
Since you're not driving a Spider in the New Zealand summer, this might not be the most useful advice for you
Now onto yours...
I think you need to replace the thermostat - the gauge should be sitting at around 70-90 degrees after a five-minute drive (maybe ten minutes in winter). I bet your thermostat has failed slightly-open as they always do. Pretty easy job to change and the heater will work even better.
As for the diagnostics, I also have the weird temperature reading (which software are you using?) - that's probably the minimum possible reading, so it indicates a poor connection, open circuit, or faulty coolant temp sender. The coolant temp sender for the engine has a rectangular two-pin plug usually blue in colour (not the two separate terminals of the other sender - that's for the gauge and light). I haven't dealt with mine yet but again, it is a good idea to, because otherwise the engine management will be running it over-rich and wasting fuel.
Plus, the engine oil will be deteriorating with the fuel that makes it past the piston rings, and the moisture that condenses when the engine cools - a properly hot engine will evaporate this, but otherwise I think you should expect to change the oil once you've dealt with these two things. Sorry if this is a worst-case scenario but hopefully it gives you one opinion on what's going on