Interesting thread, thanks for keeping it up. Lots of possibilities to run down so I can see how a mechanic might focus on past experience to shortcut the diagnosis. Let us know the final fix.
We bought Ford vans that idled terribly from new, spent a good deal of time at different dealership shops to figure out why, and finally a guy came out "from Ford" and pronounced the idle normal. When we sold them years later my brother bought one. He called the next day to ask why we'd removed the throttle-body riser, cuz the truck idles terribly without it.