Not likely as both were built on the same lines in the same plants.
The only real difference between the Duster and Demon are the front clips and the tail panel. (The left front door on my Demon came off of a Duster.) The Dart front end has a little more overhang than the Duster and it uses a lower valence panel that is not part of a Valiant front end. With the same equipment levels, the difference in weight between the two is minimal.
As for flexibility differences, it is a myth. I had heard that heavier gauge sheet metal was used in the Dodges. Also, not true. Using repro parts it is possible to convert the tail panel. I haven't seen a 72 Demon grille in repro, however.
Customized, yes; roll down, no. The rear windows in my Demon have a manual remote control courtesy of an 80s vintage Grand Caravan SE. If you prefer power operated, look for an LE model. I have not seen remote mechanisms in any SWB Mopar minivans in my sorties into salvage yards.