The "hanging rpm" may be normal. The computers are programmed to do this to help prevent unburned fuel from entering the exhaust
and burning in the catalytic converters
. The IAC is held open by the computer to allow the rpm to drop slower than it would on a carbureted car. Most people don't notice. Disconnecting the MAF may just cause the computer to default to a program where the rev-down strategy is different. Make sure the MAF is clean, you can clean it with CRC MAF cleaner to make sure:
There is some discussion about the hanging idle rpm on the Mustang owner's forums, and they have come up with the idea of restricting the IAC airflow slightly by installing a metal plate between the IAC and its mounting surface. The plate has smaller than original holes to lessen the airflow allowed into the engine, which brings the idle down faster. owners who have done this have reported more burbling from the exhaust, which is due to the unburnt fuel burning in the exhaust. The size of the hole can be fine-tuned for the car, since some cars need a little more air depending on the engine load at idle.
Here is a picture with a 9/32" hole:
You might want to experiment, but it is up to you, I have not personally tried something like this. I would check with the dealer first, and if they say they can't do anything about it, then you may want to try out the restrictor plate. There is an ebay seller also that sells a similar product, search "iac restrictor