[BusyBody]

Sorry to read about your problems. My Impala buying experience (also an 08 LTZ) has been excellent. I agree with you about some of the plastics like on the shifter, but it is overall a great value for what you get compared to other sedans in this segment. The fit and finish is excellent on my car. I found my mileage improved after my first oil change, and I am averaging about 10-11L/100 km, and I drive almost always around town.
As far as pressing the FOB twice to unlock all 4 doors, this can be programmed using the control keys to the right of the steering wheel, I believe its the information button. The default mode is only to unlock the driver side on the first press of the FOB for safety (in case anyone jumped into your car), but it can be programmed to unlock all four on first press. My saleperson asked us what we wanted and we opted for the default mode for security.
Any pics of your new ride? I hope you iron out your problems, but you should remember that no car is perfect, although it cetainly sound like the dealer you chose could do a better job at customer follow up. Our dealer has called and sent a thank-you already and our car is about as old as yours.