2005 Mercury Montego Lost seven miles per gallon

It is true that winter fuel doesn't pack as much energy as the summer fuel.
However, there are other reasons for lower fuel economy. Colder ambient temperature places a greater demand on the engine's cooling system. The cold temperatures causes the engine to consume more fuel to maintain the operating temperature. Using the heater also cools the operating temperature.
Low tire pressure, dragging brakes, out-of-alignment, air condition (defogger uses the air conditioner to reduce moisture inside vehicle), a weak or coated mass air flow sensor will give inaccurate reading to vehicle's computer.
It looks like you've maintained your vehicle well so I wouldn't suspect that there is an issue. Unless this continues into spring.
Hope this answers you question.

The electronic throttle body is all one part there are no serviceable parts and thus the high cost.
I found the customer satisfaction bulletin about the build up on the throttle body. According to the bulletin this build up was not accounted for when the programming went into the acceleration system. This issue may or may not cause the check engine light to illuminate and engine idle RPM variation. It effects the idle quality only and could decrease fuel economy but only if the check engine light is on.
I wouldn't consider the throttle position sensor as the cause of reduced fuel economy. It usually causes unusual drive ability issues or causes the engine not to start. However, this is an electronic throttle body and the TPS is monitoring the throttle motor and relaying the throttle plate position to the vehicle's computer. In other words when you press on the accelerator pedal is the electronic throttle body motor doing what the driver is requesting. If it is not it is going to flag a warning or check engine light.