I am having a similar problem with not being able to find Halsin after freeing him, not using him in my party, killing all of the goblins, not killing everyone in the grove. I did return to the grove and talked to Zevlor, talked to Rath "where is Halsin", partied with Zevlor, rested, returned to camp, return to goblin camp where everyone outside was hostile and killed me - so much for loping the head off and the rest will scatter theory. Somebody needs to warn the tieflings because they will all be slaughtered the moment the leave the grove.
