Why is doing custom head and eye textures so fucking complex lol (Default Female Ohmes-Raht at the moment)

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So I've been working on my own little (UNDERSTATEMENT) mod for a while now and I figured 'why not upload it to nexus once it is done?' It's a mod regarding the Khajiit breeds of Tamriel, as I've been bothered with seeing them underrepresented in the games (not including ESO). I plan on making this a massive overhaul that adds all of the canonical Khajiit breeds into the game, bipedal or not. I've not yet settled on a definitive name, but I'm thinking something like 'True Khajiit' will do. Of course only the bipeds will be playable because quads are neigh impossible to turn playable with my litte bit of modding experience, and I doubt that running around as a massive Senche-Raht would be very efficient for the Dragonborn. Now, I am finishing up the Ohmes and Ohmes-Raht breeds and once I got everything figured out with them, I might be posting a sort of betatest for the aforementioned races if I can figure out how all of that works.

Aside from all of this I am also here to ask for possible help. Is there a way to get custom textures for the face and head to properly function? I keep getting the same error for both. The head tex works fine now because I did some magic in Nifskope (but I still get the error), but the eye textures just don't function at all.  (I know i should probably get xEdit for this, but I simply can't see the need to install an entire developer setup for one single mod, so I'll be happy with every other option.)