Players are persistent, obviously. When you change instance, you are still there in the game until you log out. And when you log back in again, well there you are again.

Likewise, if one of us destroyed another player that we had both been chasing then once that player re-buys their ship and comes back into the game they will obviously be there again and can come back and destroy us!
If a player drops from supercruise to try and evade an attacker, the attacker can target their (low) wake and drop down and they should be in the same instance again. And obviously they would be the same player(s) as they were before.
NPC's rarely low wake, when they jump out to save their skin they always high wake (to another system). Theoretically, if you have a wake scanner fitted you can follow them, and theoretically you will find the same, damaged NPC in the new system. I say theoretically because I've tried it a few times and there was no NPC there. The only time (in my experience) that an NPC low wakes is when they are a contact that you need to find as part of a mission. I imagine that in that case when you drop down and see them they are again a new spawn of the NPC with the same name.
Basically, NPC's are simply random computer generated ships that spawn for you to populate the galaxy. Sometimes they are spawned for atmosphere, and if you have missions that involve an NPC they will spawn specifically for that mission purpose such as a pirate lord for you to destroy or a pirate trying to steal your cargo. But essentially they don't exist either before they spawn for you nor after you change instance, and as I said, if the 'same' NPC is still required for your mission, it will just be a new copy of an NPC with the old name that respawns.