I remember watching my mom play through all multiple endings at once (though apparently this memory may be false) so I thought I could get the bad ending, good ending, then go back to finish up the gems I missed and get the perfect ending. However the game is now locked to a statistics screen and I can't redo level 20 as I was hoping.

I'm not really interested in playing through the game three times, I just wanted to see all the endings. I would have just gone for the perfect ending if I'd known this would happen, instead of spending so many hours on the side quests.

Are there any files I can edit to reset the final level without destroying the rest of my progress?

I tried scanning through some of the game files but I can't really figure it out. Since all the game data is just listed in easily editable .xml files, I thought there might be a save file with variables I could change, but I can't find anything like that.