I'm using the Krait MKII as my mining ship, too. I'll try to answer your questions.
1) Limpets are little drones that go and pick up the mining fragments and bring them back to your ship. You can launch them whenever you like, they'll hang around for a while, then die (how long depends on which controller you have). Of course it's best to launch them when they have something to collect. I launch them right after cracking open the asteroid. Make sure you have nothing targeted when you launch them, so that they will automatically pick up anything in the vicinity. Also remember to BRING SOME LIMPETS. You don't want to get out there and not have any. Not that I'd know about that. I take 50 limpets for a 64 ton cargo hold, and I usually have to dump some to make room before I'm done.
2) NPCs usually spawn when you enter the ring, or anytime you log off and on inside the ring. They will sometimes be pirates, sometimes police. If they are pirates, they will usually fly over and scan you for cargo. If you're only carrying limpets, they go away. If you have a bunch of expensive cargo you just mined, however, they will ask for some of it. You can give them what they want, and they'll (usually) leave you alone then. It's sometimes bugged. Or you can run away. I turn and blow them up, but I'm geared for it.
Best way to control the situation if you DO have to log off is to leave the ring. Get into Supercruise. Log off there. Then when you log back in, you can decide to re-enter the ring (and expect pirates), or just fly to a station and sell your stuff first.
When you are on your way to sell, and you have a cargo hold full of stuff, you will often be interdicted. Not every time, but lots. I just fight the interdiction myself, because I'd rather not get into combat at that point. I'm on my way home, go away pirate. If I actually lose the interdiction (almost never) then I'll fight. If I was in a ship that could not fight, I would high-wake out (that means jump to another system...not just into supercruise in the same system). That's the best way to escape and not get caught by mass-lock effects.
Surface mining can be lucrative, but it's a whole other skill set and ship setup. Learn it later. Focus on core mining for now.
Have fun and feel free to ask more questions.