If a battle is occuring, as you pointed out, 20km from the DS, why not set traitor messages or self-destruct/warp-out cues if the player leaves the mission area? So far as I see it, the point is to keep them in a battle away from the Death Star, so that all those beautiful greebles are irrelevant (since they'd suck too many resources as it is). Though as a part of that, perhaps a long-range bombardment from the DS would make sense; during the battle, they start firing into the zone so it becomes more of a challenge to avoid lasers (and less of a chance of a player running off to see the DS).