What's the heater got to do with mileage?

I don't believe the heater switch uses the A/C compressor with the exception when you have it on defrost (even that I'm not 100% sure about in the Xfire). Unless they have changed design significantly enough to eliminate the heater rad core under the dash/at the firewall, there has never been a need to use the A/C compressor for heating the interior.

Any other comments on this?

Your right about the heated seats. They are strictly electical and would have no more affect than putting the headlights on at night.