I bought this on Steam. I can't get the game to unlock adventure mode, I keep replaying it and getting an ending, but it doesn't flag it being completed.
Complete the game with the True Ending. https://drive.google.com/file/d/1bCu0vcesAqWLzjSTjrx80l6OT6cJWagX/view
Check out Klaysee's guide, the quest starts on page 70. You need to complete that quest to get the true ending and the developer island.