No reason for storage alts to be in the game. They are annoying to play against, inconvenient to play with, and all-around unintuitive. Being able to logout 100% safely with anything in a full loot pvp game will be abused and should probably just be removed. If you want master keys to be 100% safe then make some sort of hearth magic cloud storage you can manage the keys from instead of this garbage. This would probably need a few big changes to implement properly.
My suggestion is to
1. Prevent characters from leaving the world without death (make them sleep).
2. Making sleepers movable/killable (more like banishable) but without any stat loss (outlaws not included, nidbane deaths not included).
3. Adding a login visual preview with the ability to wake up at current location, teleport to hearthfire, or teleport to wilderness (no equipment, items or curios).
Players whom logout without outlaw/redhanded become a sleeping object after their debuff expires. Sleepers can be moved, stolen from or killed (with respect to the usual laws) but NOT given wounds or items (idk why the fuck KOed players can be given items to begin with). Moving a sleeper counts as theft. To kill you would just right click>kill provided you have murder unlocked. If a non sleeper is killed, they drop their items and curios as usual but do NOT incur any other penalties or benefits (wounds, energy, lp, etc stay the same). If sleepers log back in after being moved or stolen from they are notified during the login preview. Deaths while sleeping (non outlaw/nidbane) would not drop a skull or buriable corpse. When you preview on login but do not choose any option, you are forced to login at your current location after 30 seconds. PvE/drowning does not impact sleepers (unless they login and ignore the preview).
Claims + palisades + murder protections are strong enough right now. I don't see why valuable items should be impossible to take. This is also why barely anyone bothers with siege. If you REALLY want some logout protection for items I think it should be temporary and done via some new hearth magic.