I cast silence in the area before the encounter and made sure it was still working, but she still summoned the guards. It's a scripted event, even though Silence really should work. Then I went back and tried Hold Person and she still summoned the guards.
On the smokepowder barrels: breaking them and lighting them does some damage, but the real damage occurs if you hit an unopened barrel with flame. Two of them will take out any of the goblin leaders, Gut included. You can drop them ahead of time for Gut: wait for the wandering goblin to go elsewhere, pick the room lock, and drop the barrels. When you are in the room with her, don't even initiate combat, just flame them.