What temperature for Betta fish?

Need help finding the right temperature for a Betta fish in a 5 gallon aquarium
Hey folks, I need a little wisdom if you have it. I've recently set up a new 5 gallon for a Betta on my desktop. The aquarium has a filter and it has a heater. I cycled it before adding the fish. My issue is with temperature regulation. When I added the fish, a red and white betta I named Maple because he looks like a Canadian flag, I moved over a few items from my other aquarium to help make 110% sure the beneficial bacteria would be present. The other aquarium is a 10 gallon with another betta and some tetras. I have successfully kept them alive for three years now, so I think I have a decent level of understanding with how to deal with small aquariums, that's why I decided I would be able to go ahead with a five gallon. One of the items I moved over was thermometer, and that's where I started encountering a problem. The two thermometers in the tank reported different numbers, and the third one I added was also different. I don't know what the temperature is in my tank. I tried to fiddle with the heater to find a real green zone area all the thermometers agreed on, but it seems like when I turn the heater on even a little bit all three say I'm overheating. I have since unplugged the heater, more than 24 hours ago. My thermostat for the house is set at 73, but the thermometers both show the temperate in the tank higher than that. One, a Zoo Med digital shows the temperature at 79.3. The other, a generic Pet Smart suction on the side is reading 78.

So, here are my questions for anybody with experience and wisdom on this:

1. What is the ideal temperature for a lone betta in a 5 gallon?
2. How the frig do I determine what the actual real temperature in my aquarium is?
3. Is there some reasonable explanation why multiple thermometers are showing the temperature as higher than the thermostat temperature in the room should be even with the heater unplugged?

I don't think any of these readings are outside of betta tolerance, but I kind of strive to make an effort to provide ideal conditions for my fish. I'm a vegetarian and believe in animal rights so I take seriously the idea that humans who keep pets have to do it very ethically.