Wee update. I tried changing the face tint layers to have the same RGB colour as the centre of the body tint layers. There were only four to change, but it didn't, err, come out too well. So the layer colours and indices, for the body and head, aren't indexed and/or used 1:1. Ended getting blue faced African American NPCs.
Which mesh has the fucky normals around the neck? The body meshes, or the head mesh? Knowing Bethesda, it'll be both.
Edit: Loaded up an outfit that exposes the neckline, and headhuman.nif. The normals are all over the place, the head mesh having the more wonky normals of the two:
https://i.imgur.com/RJgkOcS.png
The green lines are the normals for the outfit's neck, and the blue lines are the normals for the head mesh. The blue lines look much more random and inconsistent. What we need, is for the green and blue normals to be the same.
There is also another issue. The vertices for the neck don't line up together, and instead clip through:
https://i.imgur.com/lrzwOMs.png
The red are the polygons that are part of the outfit. You can see them clip into the polygons of the head mesh. There are two options to fix this. Either edit each and every outfit for both genders, or edit the head .nif and each .tri mesh variant.
Either way, it's going to be a shit ton of work.