When I import my hair it doesn't appear in sims 4 studio

You need to make sure all the mesh parts are connected to "rig" (and not "rig.001" or "rig.004" or any of the other rigs, or that they're not connected to rigs at all). 

Click the wrench icon for each of the groups, click below where it says "object", and make sure they're all connected to "rig". From what I can see, only the "s4studio_mesh_1" group is properly connected, and not the other two meshes. When you're done, delete the other rigs and keep only the one named "rig". If there are multiple rigs, it seems S4S gets a bit confused, and won't import the mesh properly. 

If you have a lot of recolors for the flower wreath, you can make it as an accessory (fewer recolors needed, and it's easier to use it separately). 

Using a hand accessory shouldn't matter as long as you UVmap it properly and make sure to give it proper vertex weights and all that (so it follows the head). 

Do you have the package file you're using?

And have you done vertex painting and the UV_1 map and all that? 

I tried reassigning and removing the rigs like I said above, and the mesh imported fine for me (but I don't have the pack used to make the base hair, so all the textures had deleted themselves from the package - I thought it best not to reupload the package since it quite likely had a few issues)

Fixed Blender file and a picture that shows it did import to S4S (texture borrowed from the Blender file, since everything was skin colored)
simfileshare.net/download/1959266/

I can't promise it will work fine ingame (I didn't touch anything having to do with UV_1 or vertex painting either, and didn't check it), but you can see if it works. If it doesn't, that's probably the first few things you'll want to check.

I'm also using 3.1.3.0, and Blender 2.76, so it's really strange if it doesn't work for you.

Package file:
simfileshare.net/download/1960291/
(sorry if the textures are borked, like I said it's probably because I don't have that pack, so they may be lost in translation - I added the placeholder texture to texture 11, one of the grays).

I didn't change anything in that category. Could be it's something that changes automatically with the rig changes.

I've seen other people having similar issues when they've used "append" for hair files, and it seems the extra rig files cause some problems here and there.

Maybe see if it works to export the Blender file from the Package file I uploaded, and then import it again to your package file (without changing anything)? It might work, and it's possible whatever was wrong has been reassigned and partly fixed by S4S.

Another advice is to use a basegame hair when you're making clones (extract the mesh and textures you need, then clone a basegame hair and replace the resources with the ones from your hair). It could be some EP/SP/GP hairs tend to have issues, and the last time I saw someone had this particular problem with a hair it was also cloned from an expansion.