I already tested your plugin, the latest one you both were building. I mean, I did saw it worked well with mtl and all.
with "no v. normal fliipping" pressed, the problem dissapears in what is render...
wait...it also dissapears in solid view, in th eblender viewport..other darkened areas also dissapear...(in between hair and face was black where it shoudlnt, also...)
Seems that function does no good things to the mesh, and when the button keep pressed, the model is seen good in viewport and render!

Nice find...Though I'm a bit afraid..as all what I do is mostly real time 3d stuff..I need normals to keep good values... Well, I'll be able to fix in Ultimate Unwrap, or whatever, even with bones and weights, no problem.
BTW, the model is open source (and wildly shared) so I have usually a link, though is not always there...yes it is. I think I have solved now the prob, but grab it if you want to see if there's some issue with the plugin, which I don't think it is.
it is here
sorry I have in 7z format, the open source decompressor is at
www.7-zip.org