I want to create a system where everytime an NPC (animals, dragons are not counted) is loaded, its inventory is scanned and then every supported item gets switched out with duplicates.

Now, the problem:

• How do I go about this? I remember in Oblivion I create a huge AoE spell with OnTarget script effect, this spell is cast every few seconds. Is there an equivalent in Skyrim?
  
• The above method taxes CPU, and I haven't talked about the script that will run itself. Is there any event to detect when an NPC is loaded without attaching a script to said NPC? I want to captured dynamically generated actors as well (like the Thalmor agents)
  
• Is there any way to determine if an object is being worn? Especially if there are 2 of them in the inventory? Like, say, a bandit has two iron shields but only wear one. Best if it's native (doesn't require SKSE but I don't mind)