It seems like you have the best advice so far. As long as the engine is running smoothly, there should be no other problems.
1) Correct your mileage calculation- use the method BCZX3 suggested.
2) The ECU reset shouldn't be done without doing the A/F reset at the same time. This requires you to idle the engine in P or N for 5 minutes after it is dropped out of high idle mode. When the test is finished, the idle will drop slightly. With her short driving- it might not have reset the A/F yet. I'd go out and start the car and let it idle for 7 mins exactly- TIMED. You can use the electronic odometer trick to see if the idle drops slightly after 6 mins or so. That will indicate it's done. I'd expect idle to be around 1100 for a car that has not passed this test, and 800 (IIRC) for an AT in N. It might be lower, but that is not bad unless the idle is extremely low in gear- like 500.
3) I didn't see mileage, or engine. We've gathered it's an AT, but we need to know the engine. Mileage is important also because if you're approaching 130k+ or so, and the timing belt has not been replaced, that would explain some of the issues.
Honestly, with a 2.9 mile drive, I'd only expect about 21-23 mpg, and yes, as Iminhell suggested, you might be experiencing fuel degradation. If nothing else, I'd give some Sta-Bil a shot- make sure you get the gas type not diesel. Finally- your fuel filter is due for a change. Yes I know, way under the 20k mark, but I do recommend that as 20-30k or 1 year. I change the wife's yearly, she puts about 10k per year on the van. It will be dirty.
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