1. You can safely store all your items in ANY container in Castle Volkihar
2. Containers, Bookcases (the ones you need to actually open), Wardrobes, Chests (including the one in the room with the blood fountain), basically any container you have to actually open and a menu pops up, DO NOT RESPAWN, so go ahead and use them to store items, I suggest removing all the clutter of fine clothes and random books inside of them first though.
3. The display cases DO RESPAWN unfortunatly but whatever item you put in before still stays after it respawns, it'll just respawn ontop of your item
Note: If you unlock them, when it respawns they will become locked again
4. The weapon plaques and weapon racks DO respawn, BUT, only in the slot where it already came with a weapon, the empty slots dont respawn and you can use them freely. This includes all weapon racks (Blacksmith room, Coffin room, Harkons room,etc.)
Note: If you place a weapon in one of the slots that already had a weapon, it'll respawn ontop of your weapon, you can still take your item back though.
5. Anything you drop in the castle floor does NOT dissapear, so go ahead and throw skulls and hearts everywhere if you want for decoration
Note: If you drop an armor or weapon, you might be approached by someone in the castle if they can keep it or not, simply say no and the item will not be taken
6. The ONLY things that respawn are the massive amounts of goblets, blood potions, torture tools, plates, alchemical ingrediants, etc. laying around all over the castle tables, so if you accidentally used Fus Roh Dah and made a mess, dont worry it'll fix itself.
Hope you guys found this useful! It took quite a bit of work organizing it and finding out what works and doesn't.