As far as I can tell, all Camp NPCs can and must be picked up from towns (or other non-dungeon areas in which NPCs reside), but I still haven't reached the ending yet, so take this statement with a grain of salt.
Regarding the Mimic Stick, does this mean that it can be obtained at different points? If I remember correctly, I got it in Lenadia by talking to Lysha (Ami's mentor) south of the castle near the lake with Ami as the party leader, but I don't know whether this is only possible during the game's first part.