Hello! Just wanted to check if it's possible using the itch-io or Patreon version of jav-it to decrypt R18 movies that I've already downloaded to my harddisk? If so, how do I go about doing that? Or would I need to redownload my library again?

Hi there! You would need to re-download your library again, but it should be DRM free afterwards. Unfortunately, once your videos are already downloaded, there is no way to get your encryption keys.