At this point in my tweaking sims stuff, I've done a handful of hair mashups or conversions and none of them have given me the grief this particular work has. I have tried multiple times to remake it (starting from the original meshes every time) and tried applying it to different base meshes (worried that I somehow screwed up a setting). In Blender, Milkshape, even TSRW, it looks fine. But in game, it is super shiny and the lighting hits it weirdly. Every. single. time of the dozen or so times I've tried to solve this. I just don't have a clue how to fix it and suspect I'm just running in circles, obviously doing the same wrong thing over and over again without realizing it. I don't want to give it up, because the pieces work pretty well for my envisioned final product (until I feel up to meshing my own), and well, on their original versions they don't have this problem.

Can anyone help me or give me a hint what stupidity I'm committing this time?

Usually when I run into the dreaded vaseline shine, as it were, the culprit is the resources referenced inside the GEOM itself- which will be broken if they belong to one of the originals or a reference.
I'm working on Part Two of the Hair Start-to-Finish tutorial and all of these quirks get ironed out at assembly so hopefully the info you needed will be available in full soon!

Use S3OC to clone an EA hair, and Transfer GEOM Data (using GEOM tools if you don't have these yet) from it to your hair in Blender (should be fine to Transfer to any LODs, so you don't need to remake them). This will make sure any weird properties from CC hair get fixed, too.
Export as .geom.

Assemble your hair with old reliable, DABOOBS. This + GEOM Tools and you genuinely don't need to touch TSRW or Milkshape to make hairs anymore.
It will renumber everything inside your geom to match your textures. It should look squeaky clean ingame.

Feel free to upload your hair on a reply if anything is still broken and I'll take a look. It would help justify the hours sunk into tutorial writing to prove I know what I'm doing

Thanks for the reply.

I tested it after following what you said and while it isn't super shiny (at least in game, CAS was still slightly shiny), it is still doing the weird lighting effect where parts are super dark compared to the rest of the hair (not as in black, although I did get that at first, had to go in and change those settings.). This hair actively hates me. >.<

I've included the hair if you want a look. I know the texture isn't great (I was still perfecting it when I decided to try it in game and this happened), so I apologize for that.

I plan to start over again, this time following your tutorial from the beginning, but if you can figure out the problem, that would actually help immensely.

I've had a thorough look and I'm not surprised you got stuck! It's actually a very sneaky EA issue, with whatever default hair and scalp resources it grabs.
Make sure to include every texture DABOOBS asks for, even using a blank one for the ones you don't want. You can grab the halo and scalp from any EA hair clone.
For example, I used the halo and scalp pieces from afHairLongStraightPulledBack and a couple of mini blank textures for the face ones.

I've uploaded those minis to Drive, because comments don't like attaching non .package files even archived.
(BlankSpecular - use this one for Specular and Control you don't want, and EmptyOverlay - use this for Diffuse you don't want).
I've also included your LOD0 with remade undersides and cleaner normals, for smoother lighting than the one in the screenshot (it was the first thing I looked into and wasn't related to the issue, feel free to do whatever you want with it.)

Thank you thank you thank you for getting it to look decent and helping me see what my errors were.

I have actually been fighting with this hair for years(!) now, and I probably was making the same silly mistakes over and over again. I hadn't used DABOOBS in a long time (since I did my first mashup somewhere during the middle of the active development of Sims 3), and had forgotten that rule about ALWAYS including all textures, so . And I was afraid that I had somehow messed up the normals with how I had done things.

I know this game and meshing can sometimes be confusing (one time a mesh works no problems, the next time you do the exact same thing and nothing but problems), but I SO appreciate having far more knowledgeable people like yourself and the others who've helped me out to make things make a little more sense.

Normals were my first thought too! I should know better by now that things looking fine out of game is the biggest flag that it's package/game-side.

Best of luck to your future projects.
When the hair tutorial is complete, I hope it can help eliminate that unreliability from the process.