Go ahead and take it in. I'd like to help, it sounds like no one on that end is listening to my suggestion about learning the throttle stop, I still maintain that this may be the only problem. Maybe the guy doesn't understand this needs to be done when replacing the throttle motor (throttle slide=same thing, tech definition vs. part definition). When he went back to the original, it needed to still be relearned. I think he's chasing his tail now. When the throttle stop is not learned, it will drive OK, except when coasting or at idle, it won't idle correctly.
It may learn it by itself if you just turn the key to "ON" and wait a minute of 2, or it may have to be put in to the learning mode, it seems like this varied from system to system. If it's learning it you will hear a strange electrical motor sound (like a whining or high pitched buzzing) as the electric motor runs the throttle plate from one stop to the other.
I am especially skeptical because now he seems to be backing down from his assertion that the EA control unit is bad and suggesting the dealer. If he has a fault code for a self-diagnostic failure of the EA control unit, why not accept it and order the control unit? I'm sorry, I don't know if this needs to be adapted to the specific car or not, but I do know you sure as heck will need to learn the throttle stop!!! Also why didn't he give you the fault code that is telling him that the EA control unit is faulty? "You didn't see it" makes me think "He's bluffing". Good Luck!!!