Hello, again. I've go excellent news. I've attempted the "copying stuff from redist to root" solution, and it worked ! So, it seems that my boxed copy definitely uses a non-redist structure for everything.
I've tested loading, saving, various parts of the game, played the DLC for a bit to see if there aren't any hiccups... Everything seems all right. No issues with the 1.3.1 update so far, and I doubt I'll have any.
I've also fired up the editors, they seem to be running just as fine. I did some tweaking of the preferences for the LE, built a small map and I've already tested it in game to see if I run into any glitches. Not a single problem, so it looks I'm back on track with TDD modding.
1.3's fine.
Thanks for the advice and encouragement, Mudbill.
Though it wasn't a huge issue, you've really helped me make sense of the root vs. redist folder issue in my particular case.
"You... silly Billy !" (Clarence, Penumbra : Black Plague)