So, is there a mod that fixes this? Is someone working on it or is it just plain impossible?

I hope it can be fixed cause it just looks stupid that one weapons dissapears when sheathed.

Yes there's someone working on it. http://www.gamesas.com/topic/1374311-wip-arsenal-visible-weapons-and-stowable-shields/

Also, someone's already done it. http://skyrim.nexusmods.com/mods/17746

http://skyrim.nexusmods.com/mods/9614 does it also