Regarding #4 and #5...
Overinflating depends on a couple things: time of year and how much you overinflate. Tires lose air anyway, so periodic maintenance is important. If you filled your tires up in the summer, check in the winter - colder, denser air will reduce the pressure, and you should reinflate to specs. I always add 2 pounds more than recommended to compensate for temperature fluctuations and pressure loss, plus the car handles better.
The a/c on is better on the highway due to drag. Around the city, I'm convinced windows open is a better option.