Dumb problem is dumb. So, so dumb.
After "fixing" my model - I decided to start over with the arms, imported the original meshes, did a new skin wrap without chopping the arms up this time - I still encountered the highlight issues on the wrists. I barely touched the wrists that time, so it turns out it wasn't anything I broke. A surprise, to be sure, but a welcome one. It took me a while to connect the dots, and I couldn't have done it without you guys, but it finally makes sense. It seems that for the tangent space calculations, direction matters. Up always has to be up. And there's probably a very good mathematical reason for that, but without knowing that, and without having reason to check for that, it took me days to see the problem on the original game UVW map. The arm UV islands point down, with the wrists at the bottom of the map. The hand UV islands, however, point up, with the wrists and the base of each finger at the bottom. Just as the polarity of the UV islands was causing a problem on the shoulder seams, the differing orientation was causing the problem on the wrist seams.
Probably most of what you listed were, yeah. The only part I did an extensive mapping on was the torso. I un-mirrored and realigned the skin part and completely remapped the underwear part. For most everything else, I only moved the UV islands around to make room on the texture. The boots and necklace were from her clothing model.
MDLOps had other problems with it: