Ok so, not sure exactly what the issue was, but I re-downloaded the file from XBL, put it on my internal SSD, opened C3ConTools 4.0.1 in Admin Mode, manually opened the file in CON converter, then edited it with quick pack editor, and it worked flawlessly. Not sure wtf was going on, I’m guessing that my external HDD had some file exception. Oh joy, I guess it’s time to back up & reformat my external HDD again 😩

Hopefully this helps someone else who has the same problem; I nearly went mad from this issue!