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.

• 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.

• On the game’s credits, have a section thanking everyone who helped (regardless of how much).

• 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.

This is a difficult problem. I licensed a game that had no credits in the game or the packaging but did have a not quite complete list in MobyGames, so what I did was list more complete credits in the store descriptions of my version, butafter  several people complained about their titles,  I removed all the titles (mobygames asked me about that and noted some studios like Valve do that)  and still someone complained that they did all the work in their section and no one else should even me mentioned, and that they were losing out on jobs because recruiters would check mobygames and see it didn't match what they said and they were going to hire a lawyer. So I ended up removing all my credits and anyone who wants to see or add their name can go to mobygames (imdb now has game credits too).

Which doesn't help your situation except to say sometimes you try to do the right thing and people don't appreciate it (and on the other side of the coin it is true that often studio heads don't give proper credits or even use them as reward/punishment). I do like the idea of  listing everyone who made any contribution no matter how small  (and I try to also list any third-party assets, e.g. unity asset store packages, open source libs...), and now I like the idea of industry crediting stands like they have in tv/film, which has some inflexibility but at least you can point to a set of rules that you abided by, and IGDA was working on crediting standards for a while.

One other thing I do to avoid misunderstandings  or worse is not accept or incorporate any contributions to my projects unless there's a clear contractual arrangement, i.e. I'm paying them. I've made a few exceptions, such as friend or a loyal customer providing some translations, but I've encountered some well-intentioned people who offer to collaborate and overvalue their contribution (my favorite is someone on facebook who said "I'm an artist and I think I can help you" and he drew a picture of my front end menu with a multiplayer button added).