1. The reason why unique NPCs aren't "bound" by same constraints is because they have separate textures and separate models. Or, in simpler words, they exist outside of existing character creator system.

2. The reason Auri cannot change their horns separate from their face comes from an older limitation that was in place when they were created (and might be still in place in upcoming 8.0, but we don't know yet). Basically, current character creator has a limited amount of categories it can provide, and that's why Auri have their horns tied to their face, plus also why Auri/Viera/Miqo'te cannot adjust muscle level - it's replaced with tail/ears instead.

Both points you raised are something players have been talking about for a long time, and are something SE will only be able to remove if they will do a complete overhaul of current character creator. Which we don't really know if they're working on, although they did make some off-handed mentions of that in the past.

Also, you can bypass word limitation by editing your post after posting.