I redownloaded and did a fresh install of the files. The batarang is loading properly now, thank you!
The primary piece of the gun is still not loading although all the parented pieces are, the same as in the picture I attached before. I get the same missing file error. The file path is correct; the .duf is in the correct folder. But after trying to open that file manually instead of through the content library, I still get the error. It seems the file itself is trying to reference a Poser file that doesn't exist in the package.
Honestly I probably won't use the gun, but I'm curious now about why it's happening