Hmm.. I also found out that it is also possible to have more than 2 copies when you 'refresh' an area. Doesn't it seems a bit strange? Well, not strange, I know my vocab is terrible... but what I'm saying is I always thought it's going to be something like 1 area = fixed number of NPCs. So, I would feel it would be somewhat uniform throughout the entire map. Maybe I'm missing something here.
There is indeed. As Zanz stated before, NPC's will spawn, and despawn when beaten. (Btw, I kind of hope that will also make some NPC's 'rare', for example an NPC that has a certain shard, because people will search that one more often than others. Would give it a great RPG feeling.)
Building a time deck at the moment, by the way.