From within I-Tunes, is there a keystroke or command that will delete both song 'alisas' from a particular playlist WHILE ALSO deleting the actual mp3 file from the iTunes music library, freeing up disk space?
A Playlist is just a list of pointers to songs in the Library. You can delete an item from a playlist but you have to delete the item from the main "Library" to have the file removed.
In "Library" select the item you don't want, hit Delete key or "Delete" from Edit menu. You will be asked if you want to only
clear the item from the Library list or also delete the song file. This assumes your prefereences has iTunes gathering up and organizing your tunes into the iTunes Music folder.