b) After the battle has begun. No doubt the army would be hard pressed, and the appearance of the wood wights would surely take the Mage King by surprise.
c) Not unless Martel's life was threatened and there was no other option. The wood wights belong to the realm of shadows, and they don't see things they way you might.
Now we're talking battle strategy. Which would you select? Chose wisely, or it might well be the end of you in that battle.