I'm just going off of what other people have reported - high failure rates with reman AMMs.

What makes me think AMM is - it's critical to efficiency, but the car will still run relatively normally with a flakey one so that would explain why your car runs but is now 30% less efficient, remans are reported to be unreliable, and the test is easy. Disconnect it and see if it has any effect. If the mileage stays the same, it's probably faulty.

The moving idle could be a dirty throttle plate, intake leak, worn/old IAC, or related to the low mileage.

The only things that I know can cause you to lose that much mpg are:
Massive intake leak (Car would run horribly under load)
Faulty coolant temp sensor (would be running very rich if at all)
Bad AMM
Leak in the fuel system
...Maybe a failed FPR dumping fuel into the manifold, but again it would be barely running.

It's a hunch and it's a very easy test, and I've read that remans are unreliable for years. That would be enough for me to spend an hour testing it.