Assuming you have a tablet or smartphone, I would spend the $20-$40 for a Bluetooth OBD2 reader.
These work on ANY car or truck sold in the US/Canada since 1996, so long term investment, assuming you will own more vehicles or know people who do
The APPS are free, or some are $5 and have a bit more information available, but start with free.
The IAC Valve is just a device, like a power door lock or power window, it doesn't do anything unless it is "told" to do it, in the case of door lock or window a button is pushed.
If door lock unlocks but doesn't lock then could be the lock itself or the button, the device or the control.
IAC valve is the same as the door lock, in its case the computer is the control(button)
Computer monitors RPMs, and computer has "target" RPMs, cold engine, warm engine, in gear, in Park/Neutral
Computer adjusts the Voltage to the IAC Valve to let in more air or less air to maintain Target RPMs, and it is very quick to adjust the RPMs.
And computer "learns", and remembers, what voltage will set what RPM
So even with small vacuum leaks computer will still hit the Target RPM set from the factory.
This is why I think you need the ODB2 reader, something isn't right, computer may be getting incorrect data, could be RPM data or "in gear" or Throttle position
Now if you got a 3rd party IAC Valve then it could be the problem, again, if the Ford IAC valve was the problem the first time.
IAC valve is a moving part so can, of course, fail, but 90% of sensors or electrics on an engine that are replaced were working just fine, thats a DIY stat, for Pro Mechanics its 50%, lol, but I think some of that is Bill Padding.