You can edit the config defaults in the Mod Configs/Nihilism.json file. You can edit per-world configs in Mod Specific Data/Nihilism/<world file name>.json, or else in-game via commands (type /help in chat to see the list). Let me know if you're having trouble.Can the configs be edited directly or do all changes have to be made in the game via commands?
Also, would it be possible to make it so the player can add new recipes through this mod? Is that even possible to do via modding? It's been my dream feature ever since using MineTweaker for Minecraft--combined with removing recipes (and items, mob spawns, mob drops, etc.) it's a great way to force mods to play nice together if the end user is willing to put in the effort.
You can't add recipes, but it's easy to make a custom mod to do so (I could even whip one up quickly if you give me a list). I actually have an unfinished mod called Game Changer that I'd planned to do such a thing with by configuration (among other things), but due to lack of interest it's been on hold since.