It's nice to hear that they fixed your car.
But considering they lied to you (and you know at least the oil cap thing is verifiable) don't you want to make sure that the fix they did had anything to do with your problem?
If you tell us the PO### we can at least verify the code has anything to do with the part they replaced. It's always hard to diagnose car stuff through strangers online (we can't see the car) but the rest of your story does not add up for me.
How many miles did you have on the car? The first oil change should have been at 3k miles, that is a 3.6L thing. After that 7.5k synthetic or conventional oil and a filter is all that is required. If you went 6k on the factory fill, then the dealer can say anything but this:
All oil changes have been performed at this dealership. I assume they're putting in whatever the manufacturer recommends, but honestly I didn't check.
THAT makes me think you had at least one oil change so there should not be an issue. Of course so many other things come into play like short trips (running the car for less than 30 mins at a time) temperature and climate that it's possible 6k is too much as others have said too.
The only reason you would need a flush is to remove sludge which normally only happens because of abuse and old oil. Doing a flush on an old car is risky because it can break sludge loose which can plug the oil pump or oil passages (or even solenoids, meaning they could have damaged the part themselves and not fixed the reason the light came on). In rarer cases it can damage the metal parts. To do a flush in a two year old car would be rare and it would mean that you are driving under "extreme conditions."
But again, I'm not there and can't see the car. A legitimate reason for doing a flush is sludge in the engine. This would mean the oil was extremely old and the engine was abused.
Considering you live in the midwest dust and heat are less of a factor. Do you drive a lot of short trips?