I ran your example PS file through the "fixps" utility on my Debian Linux system. (The utility comes with the "a2ps" package).
$ fixps TMP20130426150418LEEEL_edited.ps >mod.ps fixps: DSC broken. gs will be asked a full rewrite of the file. fixps: making a full rewrite of the file (gs).
You are probably right about it having to do with the file structure: the "DSC broken" means the original file did not comply with the Document Structuring Conventions, making it very hard to modify it afterwards (like changing the duplex mode). When viewing the original PS file as text, it is obvious that it does not follow the DSC at all.
Attached is a zipped copy of the rewritten PostScript file that was the result of the fixps run.
The resulting PostScript file is 5x the size of the original, but should now be DSC-compliant.
Try printing it as duplex, I think you'll have more success with this one.