hey... i was facing the exact same prob.. my mac wouldnt delete a file that i put in trash from my external hdd. everytime i'd connect, the file would show up in trash and refuse to be deleted permanently.
i tried every step in the book. every software.. shredder n everything. dint work. i guess some files are stubborn.
however... try this. i pulled the file from the trash keeping the cmd button pressed. I moved it back to the external hdd where it belonged. the file went back. dint leave a copy in the trash.
next, i connected the external hdd to a windows pc.. shift+delete and the file was gone. hasn't troubled me since.
I am afraid I can't be too useful for your particular problem, but I would suggest looking into AppleScript, if you haven't already, since there is likely some way to "tell finder to tell me which wallpaper is currently displayed".