Otis,
I hope you are still paying attention.
I've been able to make your supplied ps file truly print dplex. Here is the problem, but I don't know if there is anything you can do about it, but perhaps Matti can comment.
The problem is that there is a pagesize command on BOTH pages.
If you wll comment out line 1151 as I have below, real duplex works. In PCL as well, a papersize command always causes a page eject. In this case we get the eject before the page 2 showpage ...
%2 dict dup /PageSize [612 793] put setpagedevice
Jim