Legal aspects of modding.

I think the following approach should be used: there are two people involved – one is the official creator, and the other is a person who keeps backups on their computer. If they say, "Delete it, or we'll go to court," the official creator deletes everything and shows that there's nothing left. However, the person with the backups remains anonymous, which means all the materials can be anonymously leaked to others. If we represent this scheme as an element of a function, and the mod's development as data passed by reference to different functions for processing, it becomes almost an impenetrable scheme.
But these are just guesses; perhaps there's a way to find a loophole.
On the other hand, you're not creating a new game in the series; you're improving an old one, making a mod that isn't sold and doesn't contain microtransactions, so I think everything is fine.