These were part of a set,including cars and not seperate sale. Probably be tough to find just the unpowered unit .
Scroll down to the bottom of this Lionel decsription page for a link to the manual pdf.:
Lionel Legacy Texas Special Set
The part number for the unpowered E unit is 6-34537
"On the original Lionel set when the rear coupler button is pressed did it open the coupler on the nose of the dummy unit (running in reverse) or the actual rear coupler on the powered unit?"
Yes, this is true for all Lionel E7's( and F units as well. Headlights and marker lights would also change if the unpowered unit was in the lead.