The rules should allow the airborne elite to throw their grenades into empty hexagonals and Deathwalker to shoot his explosive shell at an empty hexagonal. This would allow them to use the full range of their grenades and special attacks.
If a figure is 5 spaces away and another is adjacent to it six spaces away, both would be affected by an airbourne's grenade. Why should it be that if the figure that is 6 spaces away is not affected by a grenade if the figure that was 5 spaces away is no longer present? The same logic (with different distances) applies to Deathwalker's special attack.
I think this would improve the game, along with the changes I recomended in an earlier article regarding adjacency.
MIf you are determinded to do it however, my recomendation would be make them roll for accuracy on the throw. Say they have to roll 13, 14, 15 or higher on a D20 or the grenade scatters on hex. (roll a D6 to randomly determine direction)