I changed some in the anime group due to the fact that One Piece is a really popular series to make porn of due to how attractive Nami, Robin, and Boa Hancock are and Rei is not Your Mileage May Vary as she was an important character in making the current otaku market with her sex appeal, and Gainax taking advantage of that by making thousands of different variations of her.

Kinda the same princible as with the Bishonen Line...

Is it YMMV if it's explicitly stated in-universe?

This trope applies to non-humans as well, correct, other than the robots and zombies mentioned? Like, say, a humanoid demon with just enough skeletal differences so as to unnerve people (like longer hands and fingers, a longer spine, a slightly differently-shaped skull [deeper jaw and longer tooth sockets to accomodate fangs], and elongated foot and anklebones)? Would this trope apply to something like that?

I found an article addressing this concept while scanning the news for stories relating to psychology. I think it's worth checking out.

Is it even possible for cell animation to fall into the category. Rotoscope animation makes scene, but anime is intentionally stylized & the look is too commonplace to be unsettling.