So, a quick note about the NPC packs.
They are next on my TODO list, after zombies. Updating the zombies took quite a bit of time, because 2.x zombies use a different system for the emissive parts of their materials (the glowing green, blue, and orange parts of the radiated, charged, and infernal zombies, respectively). They also now use a "fade" property to fade in and out (you probably have noticed this with the Plague Spitter, especially in earlier 2.x versions). This required that I learn how to code Unity surface shaders which support those features, and I'm not as familiar with Unity as other modders. There are also updates to the zombie controller and animations (due to the 1-block crouch) and a few other changes on the XML side.
The situation is going to be similar with the NPCs. I can use the same shaders I developed for zombies, which should save a lot of work. But NPC Core also updated its controller, since it is based on the zombie controller, and I now need to update my own version of that controller (which adds the "on fire" animations). There are likewise a couple other Unity changes I will have to make to them, including a new physics material on the main colliders.
I think the "invalid layer index" issue is due to one of those things. I'm guessing that happens if the game tries to access an animation layer which doesn't exist in the controller, and there is a new NPC controller. I have also heard reports that 1.x NPCs tend to get stuck on things, and that might be due to the lack of a 2.x physics material on the main collider.
I should be done with my zombies soon (hopefully), and then I'll start updating the NPCs. I also have some personal stuff going on (family health issues, my own health issues, and of course my day job), so have patience.