Something to consider in respect to why Chad and Ron Jon didn't recognize Zuko and Azula.. The fire nation is obviously a very honor (shame) based culture. I doubt that they would draw Zuko with his scar even after the incident, so only people who saw him or heard the rumor would know exactly what he looked like. Also, you would think that in the fire Nation many people would have burns from various incidents. As for Azula, I imagine it would be quite difficult to draw Azula to match her disposition. They would probably imagine the fire nation princess to look less psychotic like she acted after their volleyball match.
Actually there was this painting of zuko and mai in the black sun episode:
In a pre-printing press pre-information age I can see how some children of the nobility would not immediately recognize Zuko and Azula. Particularly since their presence was rather low-key. No entourage, no palanquin bearers or imperial guard, no distinctive royal headpiece. Even the boat they arrived in was tame.