Does a unit with scout have to attack for it to work, or can the scouting unit defend, survive, and then scout? Also, it says that the scouting unit must survive combat for scout to work... I would assume this means that if a scouting unit in the kingdom does not defend when the kingdom is attacked, scout does not trigger, correct? Thanks.

The unit has to participate in the combat as attacker or defender and must survive the Apply Damage step in order to trigger the scout ability.

bitva said:

Yes, Scout has to participate in the combat as either attacker or defender and survive for the Scout to kick in.

Thanks guys! Even though the rules don't say so, I extrapolated that a scouter had to be attacking, not defending to trigger. Glad I know better now.

If unit(s) with scout attacks and the opponent does not defend (or doesn't have anything to defend) then will it still work? And even if there is no damage applied (due to card effects etc) will it trigger the effect as well?

Yes as it still survived the combat. That's all the scout needs to do is participate and survive.