Well, it turns out that just now—after reading your comment and thinking about it a bit—I tried something new:
Instead of naming the edited file "main modified" (or something similar), I simply named it "main", like the original one.
I had saved the original file in a separate folder, and then replaced it by deleting it from the data folder and leaving only the new file named main in its place.
Just doing that fixed everything! The game no longer shows the corrupted file error, and the changes are applied
What’s strange is that my friends didn’t have to do it this way, and I’d seen some tutorials where people do rename the file to things like “main modified,” so I didn’t think it would be an issue... but hey, as long as it works, I’m happy!
Thanks so much to everyone for the help!