You should be able to use a TextureSet in this situation. Basically the TextureSets allow you to have multiple textures for the same nif, without the need to actually edit the nif.
Example: I had the Solitude banners (SBannerMid01) around my castle but I wanted to have my own emblem on the flag instead of the Solitude wolf head emblem. If you take a look at the edit window of the object you'll see the "Model" section where you can specify the nif file to use. Click the edit button next to the box and you should see the model in the preview window and on the side you should also see a section called "Alternate Textures". It is here that you will assign a new texture which will be used by this object only.
To make a new TextureSet go to the Object Window>Miscellaneous>TextureSet then right-click and make a new one (or duplicate an existing one). You should now get a window that will allow you to assign Diffuse, Normal/Gloss, and all the other texture maps for this TextureSet. Once this new TextureSet is created go back to that window where you assign the nif file. In the Alternate Textures list right click on the first line and select new (you may need to delete first), a list will pop up and you can select your new texture and see what it looks like in the preview window.
This should solve your new head texture problem. As for the missing body problem, I don't know too much about that but I'll suggest you take a look at the NordChildRace and try to reverse engineer where the body part data comes from. You might need to make new copies of the body parts and assign them to your new race.
Hopefully this helps!