To answer various questions in the thread:
NPCs always high-wake. The game has a very poor record when it comes to persistence of NPCs, but if you scan the wake and follow the NPC to another system (following a wake is done by activating your FSD with the scanned wake targetted), the game will remember their state (name, ship, damage, cargo etc..). Recently Frontier has fixed the alignment of FSD wakes with the skybox, but I doubt the game will persist the NPC across instances if you don't explicitely tells it to (by jumping via the scanned wake) and I'm not able to test that atm. Note that for mission NPCs, the NPC may no longer be a valid target mission in a different system unless Frontier have fixed that since, so make sure to double check.
Now regarding mission NPCs, if one escapes and you can't follow it (say, you have no wake scanner), you can make the NPC re-spawn by going back into supercruise rather than following the NPC into hyperspace (if it doesn't respawn, try jumping out of the system and back in). For all intents and purposes this will be a new NPC. It will not keep any damage you have previously done it it, and I wouldn't be surprised if the NPC can fly a different ship entirely. This is just one way where the game fails wrt NPC persistence as Frontier has to explicitely deal with it on a case-by-case basis. You'd think by now they would have added more persistence to mission NPCs but ah well.