Going to give you a little insight into the pre-existing materials clause.
The actual intention of the rule is to allow people to use RPG Maker DLC packs, or use royalty free music or things from resource showcase threads here or Yanfly plugins. Stuff that is publicly available to everyone and not made specifically for the contest.
Of course, because of the way we worded it, it will also include any materials you made, as long as you can prove that they weren't made specifically for the project you are currently working on. In this case. This wasn't the original intention of the rule to be honest, but I realize the way it is written it is the letter of the rule, so that is what we are going with. Remember that you have to be able to prove you worked on it for another reason to use them though!
You can prove this by showing a previously released and DIFFERENT game that contains them. Or that you released them in your own resource showcase. Or that they are sufficiently generic (such as a generic, improved dialogue system plugin) that it is clear it could have been made with any project in mind. Or that you have posts on blogs/forums/social media that predate the announce showing those resources being used in an entirely different project.