Hello and thank you for using just answer. Let me clarify a few things for you here, first the spark plugs- this vehicle uses IRIDIUM spark plugs, the service life on these are 120,000 miles so if you have never changed them then yes it is time to replace them however to determine that this is the reason that the car won't start the mechanic would need to perform a spark test, it is very common on this engine to have ignition coils go out if the spark plugs are not properly maintained/replaced at the specified intervals however I have a hard time believing that the spark plugs are the cause here unless they are completely fouled out AND they should have given you some sort of warning such as misfiring or sputtering and lack of fuel mileage.
As far as the IAC valve, I can see this as a relative concern, especially with that mileage however you should be able to push down on the accelerator to compensate for a sticking IAC valve. All the IAC does is control the idle speed, it does the same thing as if you press the gas pedal, it can raise or lower engine speed. What I am trying to say here is that if the car won't run but you can keep it running with the gas pedal down then your IAC is a problem however it will not keep the car from starting.
It takes 3 things to make a car engien run 1- fuel 2- compression 3- spark. These are all computer managed on your vehicle so several other things can cause issues but these are the 3 basics to check on ALL cars when you have a no-start situation.