See this current thread about the seat heat, including my (positive) experience. https://g35driver.com/forums/g35-sedan-v35-2003-06/91829-heated-seats-useless.html

MPG is always difficult; I got 21-23 on a highway trip recently and I get about 15-18 in suburban driving (short trips). I don't think you can expect great mileage at 70-75 MPH though-wind resistance is pretty nasty.

I'm sorry you are not happy with your car. I like mine a lot (but not enough to buy yours!).

Good luck.