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Since it sounds like you make a lot of short trips coupled with a colder climate, your car may not be running long enough to properly charge your battery. This could be especially true if your battery is getting old.
While this probably wouldn't directly cause your mileage issue, it's possible that it could explain some of the weirdness with your ECU.
If your CPS is bad (or the ECU thinks it's bad), then the ECU may have retarded the timing to some default setting, which is safer for the engine but leads to poor performance and efficiency. If I remember correctly, the same holds true just about anytime the CEL is on.
Not an expert by any means, but maybe some of these could be contributing factors. Someone suggested a 100 point inspection. As much as I hate to feed the dealership service beast my hardearned dollars, that may be the best way for you to get a definitive answer.
Good luck!
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