So I've been testing the hair pieces myself and while I'd never seen the disappearing hair before, you're right, I get the same results. Your assumption may also be correct so let's talk about the grossly high poly meshes.
The main hair piece itself is 45k+ verts. Each hair part piece is 32k+ verts. Even the popular KS Hairdos are on average less than 9k verts. See where I'm going? When you add up the verts, one of your hairdos is 173k+ verts in total, and the other is 141k+ verts. It's no wonder the engine is saying 'no way'.
You need to get those meshes way down in vert count. You also need to split alt part 2-altpart.nif into 2 meshes. You can't have more than one BSTriShape per mesh for these. In fact, you should look to split all of your extra part meshes down into manageable sizes.
Here's some other tips;
- There's a ton of overlap when you combine the meshes into one hairdo. Start by deleting verts and/or whole pieces that will be covered by other pieces.
- There's a ton of verts sitting inside the head. Start deleting them.
- You don't need most of the geometry underneath each piece, just the surface, so start deleting from underneath. Try to only keep what will be seen. This is a common practice similar to making armor where the body underneath the armor is deleted.