Just assuming, you are right (I free him, when the dealers are dealt with normally and he tells me a whoel different story about finding the entrance to teh Underdark), it just would show even more, he is pretty incompetent. "Yeah, I just stay prisoner, until someone, who might never come, deals with those pesky leaders." He had no way to know, that Tav/Durge would come in with the tadpole gang to help him out with this.
*Sigh*
And AGAIN I'll mention, if you go with Minthara to attack the Grove, he also will free himself, take out Dror Ragzlin and Priestess Gut and return to the Grove.
Meaning he capitalizes on their forces massing outside the Grove (Thus not being in the Goblin Camp) to take out their leaders.
Showing that he's waiting for an opportunity to strike, whatever it may be. Whether it's someone coming in and attacking or some mobilization of troops.
The only fault with this scenario is that he arrives AFTER the battle for the Grove is over, so he doesn't protect the Grove when he capitalizes on the lack of force in the camp. But it's not as if he has much in the way of options. Especially since as long as the leaders exist, attacks on the Grove won't stop. So it doesn't matter if he's there to stop 1 attack, if he doesn't take out the leaders there'll just be more and more.
Also, "Warn everyone"? Warn everyone of what? They ALREADY know about the Goblins, that's literally the reason why Kagha is doing the Rite of Thorns and why the Tieflings are seeking refuge...