Thanks for asking.
Currently, giving a subsequent order to a unit requires you to re-roll that unit's command die and getting a higher result. There will be rules for assigning additional command dice to guarantee a unit perform a second or third order, but those haven't been added yet.
A unit is on the crest of a hill when it's touching or overlapping the terrain card. (Basically, imagine the terrain card as the slope of the hill; everything outside the ring of terrain cards is "ground level" and everything inside the ring of cards is the flat top of the hill.) The intention is units on a hill (so inside the ring of terrain cards) but not on the crest (so not touching or overlapping the terrain cards) can't be shot at by units on lower ground. If you stay away from the edge of a hill, you can't be seen be people on lower ground.