So I've created a set of weapons - simple, retextured elven shortsword, bow, and arrows/quiver. I haven't swung the sword at an enemy yet, but this thread isn't about that.

I've fired the bow three times - twice, when using my custom arrows, it CTD'd; once, when I used a mod's arrows, it fired, but the arrow dissapeared (possibly through the floor. I was in the testinghall)

Also, when I'm in he journal, the weapons are invisible, save for a silhouette behind the entirety of my character... as in, if I turn her while holding the bow, her hand is curled around air and a black shape of the bow shows up behind her.

I don't know what's causing the problem... I was hoping someone could help out. Any help is appreciated!

The black shape behind the character in the inventory gave it away.

You need a normal map for your textures.

So..........
Be sure you have a (item name).dds, AND an (item name)_n.dds.

Without a normal map the inventory oddity you described will happen. Most likely the cause for your CTDs also.

Is it okay if the normal map is blank? And, do I have to link them to the meshes in NifSkope? I don't know how to do them... I'll try adding them, though.

Thank you so much!

Nvm, I just added the default elven normal maps and it cleared up everything ~ Weird, huh? xD

Again, thank you so much!

Not weird......required by the game.
For information purposes:
A blank Normal map will not work.
They do not have to be linked to the .nif (i.e. a separate line in the .nif) since the game automatically looks for _n, and _g extensions to .dds files (They can be linked though).
I use the nVidia DDS authoring plug in for PS to make mine but I believe there are other ways as well.

Using the existing (stock) normal maps will work fine if the pattern is the same for the re-texture as it was for the original. Where you'll run into issues is when you remove/modify part of the texture that is height mapped through the normal map. It will still appear to 'stick up' in game on the parts that were once textured that are now a flat color.

Have fun modding, and glad I could be of help,
Nico