OK, I'm confused.
So the cell phone right at your ear is worse why? Is it that the "evil" sound waves lose their potency when they travel some distance through the air? Not a very compelling argument.
That wasn't the point. You're concentrating on doing several things--one of which is listening--as opposed to just listening to a radio.
Duplex vs simplex - a complete red herring. The studies show clearly that even listening to a recording with the knowledge that you will be asked about it later causes cognitive distraction. You surely cannot be seriously arguing that needing to press a button to reply is somehow LESS distracting.
Not at all, and not a red herring, since the study you referenced is about remembering facts rather than having a conversation. Now, if you're talking on a cell phone and start having a heated discussion, you're going to concentrate more and more on that discussion rather than on driving--and that could and does become two way with both people trying to talk at once. Granted, you can have a heated discussion on two way radio as well, but it won't be the same since you can't talk--or argue--until the other person lets go of the button on his mike. If you key and start talking, he won't hear you--and you know it.
Granted, that explanation isn't scientific--all too much these days 'scientific' explanations are applied to what should be recognized as common sense issues--but it sure as heck turns out to be what does go on.