I totally understand the first two points which are true and sad but concerning legal point, it's not illegal to fork the game and add more content so long that (taken from the github repo license)
License:
Redistribution, use, and modification of the Project and its Content, with or without modification, are permitted provided that the following conditions are met:
- The Project must retain the above copyright notice and this license as is.
- The Binaries must reproduce the above copyright notice (i.e. leave Innoxia's name in the credits page).
- The Project, its Content, and Binaries:
3a. must carry prominent notices stating it is modified, and not endorsed by Innoxia, if applicable.
3b. must be available free of charge, if distributed.
3c. may not be used for commercial purposes.
Points 3b and 3c are to protect the player base from having to pay for modded copies of the game. I don't want modders to try and take advantage of players (by adding a paywall to their mods).
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meaning that as long as you keep the game free and open source and warn that the game is modified and include the name of Inno you are safe to do as you wish.