My temperature liking is also between 78-80 degrees.
As for the heater, the reason why it's doing that is because your water and room temperature are probably hotter than the average room temperature. Your heater heats based on what the room temperature is, so if it IS hotter, than the temp you set will be a couple of degrees off. Try playing around with it so you can get the correct temp. It's probably not your thermometer.
Walter, just like many other betta fish, is a picky, picky eater.
If you want to get him to eat more pellets (since they are more nutritous) try soaking them in garlic juice. It'll make it more appetizing for him and the garlic is good for his immune system.
The betta guy at petsmart is right (for once lol). Bettas love fruitflies and other tiny live insects, you just need to be careful that he doesn't get parasites from it. I'm sure he won't if you only feed it like once a week as a treat. But he'll LOVE that. I was thinking of catching mosquitoes for Finn since we have ALOT around here in Ontario. The little buggers love chasing me for my sugar blood, so that would be my revenge lol.