1. You have to wait for a certain number of turns. The manual states the number, but I cant recall it right now (I guess it was half a year, i.e. 4 turns).
2. ? (I doubt it, except for well-prepared necromancers, of course...
)
3. Yes, but that depends on your nation. Most nations seem to have 2-3 national heros. (I'm not sure about the chances of national heroes to show up, but I vaguely recall that someone said that the chances are increasing each turn and are reset when one of the heroes appears.)