Just picked up on this thread and I've got a couple of thoughts.
Idle speed - It should be around 1400rpm which I think is maybe a little high. With the non-standard kit on my scoot it sits at around an indicated 1100rpm which "sounds" right to my ear. Easy check is to make sure there is some free play on the throttle on tickover. If not, the idle speed may have been messed with by adjusting up the throttle free play too tightly on the cable. You can access the adjusters by lifting the little carb flap found inside the seat bucket and a couple of 12mm spanners (from memory) should do the trick. The ECU sets the idle speed and is best left alone.
Whacky temperature reading - could be a number of issues such as:
Blown head gasket - possible but you'd be unlucky to have this and the vid I saw on YT of you filling the pipe looked inconclusive to me. There were some bubbles but these could have been as a result of an airlock after you filled it. Was the level down on the expansion tank?
Temp sender on the fritz - entirely possible. Could be the sender but could also be the earthing terminal that attaches to it.
Instrument binnacle on the fritz - again possible but less likely than the temp sender.
The extra hot exhaust could be that your valve timing is off. Simple enough to adjust up the valve timing.
Working on AvE's Principles Of Troubleshooting:
(1. Do the easiest thing first. 2. Don't rely on the people that tried and failed prior. 3. 90% of problems are between the driver's seat and the steering wheel).
I would be looking to swap out the temperature sensor first as its the cheapest thing to have to replace and may fix your dodgy temp reading. I'd also replace the sensor earthing terminal. For the sake of a couple of bucks, its not worth mucking about. I'd also check/adjust valves. Even if they end up not needing it, you'll then know when next they need looking at according to the service interval. If they did need adjusting, you'll have done it.
Parts:
Temperature Sensor 37750-PAC1-005
Cover gasket 12206-LBA2-E00
Earth Terminal 19300-LBA2-E00
I use these guys:
https://www.bike-parts-kymco.uk/kymco-motorcycle/500-SCOOTER/XCITINGAs Stig suggests, subscribe to Cyclepedia for a full manual. I did and it was money very well spent. Saved me a fortune.
Bon Chance