You need to copy pbimageviewer.app to both the system/bin/ and the applications/ folders. The one in applications/ is only for viewing image files inside a folder (it cannot be used for opening archive files like CBR or CBZ). The one in system/bin/ is what the Library application invokes when it opens an archive file. So, just copy pbimageviewer.app again to system/bin/.
The /mnt/ext1 and /mnt/ext2 prefixes refer to the full paths on the ereader itself (the first is for the internal memory, the second for the SD card). Those parts of the path don't show up when you mount the filesystems via USB. Sorry for the confusion. I'll fix up the instructions later.