My successful recruitment was as follows:
1.Liberate Halsin from the wolf pen and kill the goblins in there. (Don't let anyone escape from the room to raise the alarm)
2.Then tell Halsin to stay put. Don't let him join your party.
3.Go to Minthara and tell her you know the grove location.
4.Prepare for the grove attack and betray Minthara. Knock her out during the grove fight. Kill all the other goblins but leave her lying KO. (You can loot her)
Then return to the goblin camp and mop up the other leaders and all remaining goblins. (You can take a long rest before that)
I found Minthara at moonrise towers and could liberate her and escort her to my camp. However, I encountered a bug here. So make a save before you start the conversation with the interrogators in the dungeon, so you can restart from there if necessary.
When I took over interrogation to converse with Minthy (mentally) and then killed the interrogators. Minthara became totally unresponsive. She just stood there, did't move, could not be talked to.
When I restarted the cutscene with interrogators and told them to liberate Minthara and then killed them, Minthara was accessible and could follow me out of the dungeon.
