I think your statement: "since there are no adjustments to be made in the rear, and no bushings, mounts are bad, or members are bent, how could the rear be doing this to that tire? At this point I believe that the inboard edge was worn significantly while on the front and then normal wear in the back took it the rest of the way past the tread. " is exactly what happened.
I had a similiar situation happen to the 300E. After a "good" alignment (this is after several &^%^$ ones) I rotated the fronts to backs. Suddenly, about 3K miles later I noticed the rears were just about gone. I was shocked because the (now) fronts looked great.
I talked with my local tire guy and he said more modern tires do take on a "set". Meaning if they a worn in a location that contributes to a particular wear pattern, the tires will do remarkably well considering. But when they are put on a different location with a different alignment (like the rear) they will wear much more rapidly. Sounded logical to me.
'03 E320 Wagon-Sold
'95 E320 Wagon-Went to Ex
'93 190E 2.6-Wrecked
'91 300E-Went to Ex
'65 911 Coupe (#302580)