Assuming that everyone who has contributed to your project are not under any contract, and they did so just to help, there are things you can do to thank them.
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Personally, I would ask them for a link to a social profile of theirs, and in a page where you have information about your game, have a section, something like “Special Thanks”, or “Contributors” or something like that, and include their name and a link to their profile. This would allow users to learn more about contributors.
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On the game’s credits, have a section thanking everyone who helped (regardless of how much).
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Make cameos of contributors. Maybe an NPC is named after someone who helped, maybe a painting on the wall was chosen from a contributor.
At this point, keep in mind I’m not a lawyer. If this is your game, and it’s not open source, it’s your decision who gets their name on the project, unless you have a contract that says otherwise. If you have published your work first, before other malicious users have, it’s easy to prove that you are the original creator. If that “official novel” you mentioned, has the same name as your games, but is published after them, you can possibly take it down, as you hold the copyright since you published it first.
Don’t let people bully you into abandoning your project, you’ve worked hard for it. It’s best to make an agreement with people that contribute to your project. It doesn’t have to be a contract, it can be as little as “You help here and there, and you get your name on the credits” and that’s the end of it. At least this way, people’s expectations will be where they should.