Check if custom entity models are enabled in your video settings.
This pack now works in Minecraft 1.13.1! You'll need the latest Optifine pre-release in order to use it.
I've also uploaded an alternate version of the pack to the curseforge downloads page that has 12 halloween-themed skins inside!
I don't think this possible, but I think it would be a really cool idea to make this work with the upcoming "better villagers mod" from the What If series.
Is it possible to replace the terrible EHMM sound they do with Nya! and the models with catgirls using this?