I figured out the backup files folder pretty early on; it's saved me from a recurring crash-on-load error, when I was able to just pull files from the backup and replace the entire directory, phew!
What I do now is, I copy all the files fresh from the backup folder into my own folder somewhere else, go through that personal folder and delete any files that look like they aren't going to be something I'm interested in and/or capable of editing, and then I split the files I do want to edit into different folders as separate "Mods" (since that seems likely to be less error-prone than my previous attempt to Do Everything All At Once). E.g. one mod just to update the gendered language in that little exchanges-dictionary (all the $sir and $master and $brother stuff). Once I've edited the files there, I go copy them into the Mods folder in my AppData/Roaming/Strive/Mods folder, so I'm not editing any of the backup files directly at any step (unless I seriously screw up my process and accidently save the wrong file, which I've done at least once but did recover from). I could reload the entire game at some point if need be, make sure it's all fresh. In fact that might be advisable. Hmm.
I've never had any problems with Notepad++ -- aside from occasionally winding up forgetting which version of a file I've opened and thus editing the wrong one, but I've gotten better at being careful with that. The code is definitely getting to the right StatsTab file (or it wouldn't come up with the extra button in the first place), and I'll check later to make sure that the other file is likewise properly updating.
On your advice, I went ahead and got that Godot engine editor (two copies -- the last stable full build (not the current dev/alpha build) and the highest final number on the one you recommended for this game particularly. That's (checks) 4.3 and 3.3.4. Being able to load up the game and look at the behind-the-scenes details was five or ten minutes of pure excitement-joy as I went through seeing all the screens. It did kinda spoil a later plot point because that's the text on one screen, but eh, I knew I was gonna get spoilers due to messing with the code to begin with.
At this point, this feels like a "this step is beyond my skills" roadblock, which at least confirms I'm not missing something that would be obvious to veteran Godot coders. When I first went to college (some 25 years ago), I signed up for Visual Basic because I couldn't make anything work, despite self-teaching myself from GW-BASIC through QBasic. Turns out I'd missed the dot-operator (the object.property thing), and once I learned that, I could make everything work and basically didn't get all that much from the rest of the class (which was aimed at newbies, while I'd been coding since like 14). Knowing that the block isn't something that basic is a relief.
I think what I'll do is step back slightly, content myself with figuring out how to mod the code that has been working, continue exploring as I go, and probably change the "call me Master" button to a slave name change button, since in fact I was able to make that much work (losing a function I don't really care to make use of anyway). Or maybe I'll see if I can replace the "Release" button that I'm always afraid of pressing, since I don't really intend to use it.
I appreciate you taking the time to work through the troubleshooting with me; that has reduced my frustration back to a much more manageable level. And hey, getting me over the hump to actually downloading the editor might wind up with me getting back into hobbyist game design, who knows? Maybe that'll go somewhere interesting.