So I was wondering if I could store items like weapons on npc's and leave them to their business, without lossing my stuff?
Inventory is a tricky thing. It depends on the NPC, and what the NPC is doing.
- Non-follower NPCs can be reverse-pickpocketed or pick up items you drop, or be sold items. These items will stay in inventory temporarily. If anything resets the NPC's inventory due to a scripted quest, time passing without you in the cell, or in some cases, interacting with other NPCs, items can leave inventory forever and be unretrievable. These NPCs are no more containers than the containers in their homes, which are subject to cell resets.
- Follower NPCs will maintain the items in their inventory most of the time. If they are dismissed and allowed to sandbox, it's possible for them to lose inventory (although very rare and usually related to a quest tied to the companion). They will not lose equipped inventory. They aren't supposed to lose any inventory in this way, so if it happens, its likely a bug.
- All NPCs will use potions from their inventories, and their use of enchanted weapons does discharge the weapon based on their skill with the associated magic type.
It is important to note that an NPC may not always be the same NPC. An example is a quest in the Interesting NPCs mod where a woman wants to become something powerful. If you help her, she changes into a monster. The monster is still the "same character" but the inventory changes because the actual NPC is what changed.
TL;DR: only store items with your followers or containers you own that aren't subject to cell resets.
NPCs that don't respawn will keep items forever, anything you give/reverse pickpocket or they pick up. Respawning NPCs will lose items even if they're you followers, such as dark brotherhood initiates. Items traded to merchants will also reset as they're in special containers rather than merchant npcs' inventories.