Apologies, didn't read everything. In a hurry and shouldn't be on here but....
I'm a qg18de. The coolant sensor will read hotter than really is if burned by air or old and faded. A resistance test will still read as a good thermostat if it was burned by air.
1. Mixing coolants that reached their engineering standards through different chemicals will create conflict in performance & lifespan. It can also create byproducts or cause the coolant to gel up if you really screw up. Use the exact coolant every time or flush the system with distilled water first until it runs clear. Then add coolant, drain what you added as the water in the system diluted it, and then add more.
2. I am against chemical flushes and Prestone products but their flush is just citric acid, <-the acid in oranges, lemons, etc... I wouldn't do it in my motorcycle; however, your car will be fine. Don't mix it with coolant in your car. The acid is for the garbage on the engine. Mixingbit with coolant is silly. Mix it in with a distilled water flush. Get the engine up to temp. With healthy fluid intervals this shouldn't be necessary but after a rebuild or bad mix then it might be.
3. Asian cares don't like silicates in their coolant. This is why universal, Prestone, Prestone universal, and various brands that aren't regional, make, model, and year specific are not good coolants. I use Pentofrost coolant.
4. Coolant like brake fluid is hydroscopic. It absorbs water once opened. It starts to age. I keep any leftover in a container if I need it and come 2 years or 30k miles I drain my engine. I flush it with distilled water. Then add the old stuff still in the bottle as it will get mixed with the distilled water remaining in the engine. I will drain that off and now there is slightly diluted coolant but coolant in the engine. From there I add all new coolant. If my heater core blows hot in the summer then I know my temperature sensor isn't trying to cool the car thinking it is hotter than it really is. I take an OBD2 reader and make sure the engine temp is in spec while sitting, and I make sure the fans turn on to disperse the heat.
4. Your dash sensor and your ecu sensor are usually two different sensors. Your dash could be lying to you. Both could actually.
5. Tricking your sensor with a resistor is silly and reckless in my opinion unless designed for it. Changing coolants as if it matters is also ridiculous. You need to use a coolant design to protect you engine and transfer heat. The ecu manages the temperature and unless you code the the ecu/ecm on a dyno and redo the fuel maps and temps then other mods don't make a lot of sense unless you are ok with mild gains at the cost of wear and tear.
6. Probably have other stuff to say, but really need to pack. I have 5 days of travel for work. Let us know how everything is working out and your engine number. I'll try to help more later if I remember to go through previous posts assuming you haven't already fixed everything.
7. You replace the dash temp sensor not just the ecu/ecm temo sensor? Sonsors that work on resistance can read hotter or colder if broken depending on the type of resistance sensor it is. The engine sensor will think it hotter than it is as it deteriorates. This is a safer design. You engine may not perform well but you won't burn up the engine. If your ecu/ecm temp sensor works fine and the dash sensor reads hotter than it really is then when your engine regulated by the new temperature displays a dash temperature it may read a lot hotter than actual temperature.