“How can I avoid the late game-ending bug that stops progression?”
The one that I’m aware of involves Nowa being unable to sleep, and needing to talk to people around the castle. If you go to your room before talking to everyone, a bug will prevent the game from continuing. So, make sure you talk to everyone you possibly can during that night.
Suggestions
If your map tells you to do something at your castle do it immediately. Do not do any exploring unless your map tells you your objective is outside the castle. That should prevent it I think…….
But you better make extra save files. If you head into a certain forest Eldroad near your castle and things start happening there and beyond that seem not quite right check your map. If your map still says go to your castle 2nd floor but nothing happens up there you have the glitch and your game is ruined.
When you are in castle, always save first before entering your room. Entering your room starts the bug in some part near the end of the game.