Update: I managed to get this figured out. It turns out I had to change the update version to 00 00 AND completely blank out the update in the file. Should be good now! (Hopefully.)
Yes, I’ve actually had to edit this file numerous times to downgrade the game after updates—but every time after I edit it, it always starts preparing an update when I try to launch the game.
This issue has been killing me for days, so I decided I'd come here as I've seen some stuff similar to it, but never been able to get it to work myself.
I'm a Splatoon speedrunner, and one of the things I need is to keep my physical Japanese copy...