I don't know if this is excessive paranoia, but they are really my questions:

• Can any NPC pickpocket me for items/coins? (Town folks, Thieves)
  
• Can any NPC steal my Horse?
  
• Can any NPC break into my House?
  
• Can any NPC steal items/furnitures from my House?
  
• Can I lock my own House?
  
• Do harvested items respawn/reset over time? (Farm crops, Fish)
  
• Do NPC House items respawn/reset over time? (I've looted every single item from Alvor's House in Riverwood, but after 25 days his house is fully furnished again)
  
• Do items that I drop/store in NPC Houses disappear over time? (I left a Torch in Riverwood Trader and Lucan Valerius told me to be careful it will be gone, was he just kidding?)
  
• What condition determines whether an NPC can/cannot die? (During fight with me and/or dragons)
  
• Why does my Follower, Lydia, keep coughing & snoring? Can I check Magical Effects on her? How do I know if she has contracted any disease?

No NPC can steal anything from you. The closest thing to breaking into your house they will do is briefly follow you inside if you enter while they are talking to you, and they will let themselves back out, without any of your belongings. You cannot lock your house. No one will steal your horse, though it's possible for them to kill it I believe. Most wild 'crops' will respawn eventually, though sometimes taking a very long time to do so. Thus far I do not believe that farm crops respawn. Locations tend to respawn their contents eventually, save for any important/unique items. Any item you leave lying about or in a container that does not explicitly belong to you has a tendency to disappear. An NPC is immortal if Bethesda flagged them as essential. They will always be essential no matter the conditions. Your companions are unable to be killed by NPCs, but can die if you strike them. As for Lydia, welcome to skyrim, where many odd and wonderful or terribly annoying things happen for no good reason.

the anoying thing about your comapnion is it doesn;t mater if you did the kiling blow or not any scratch you give to your companion means they are killable

Thank you, Olfheim & Viglante.

#1-4 No NPC can do any of those things.
#5 No, but since NPC's don't do any of the things in 1-4, it doesn't matter.
#6 Yep, after ten game days
#7 & 8 Yeah, after 30 game days, both things happen.
#9 NPC's cannot die if they are considered essential by certain quests, or if they are your follower
#10 No idea, but I'm pretty sure not. Can't help you with the snoring, either.

Only the kids in The Den. Regarding your NPC coughing/yawning/etc, they do that during different times of the day, like yawning at night or just randomly sneezing so that you know they're there and so they aren't too quiet. I'd really like to have one that speaks more often.

Hey, which Den is it? I can't seem to find "The Den" on the Wiki :S
There are so many Dens: Swindler's Den, Den of Thieves, Bravil Skooma Den, Rotten Den, Mara's Eye Den



Thank you, SgtScooter. Your help clears up all my questions.