While trying to free the Owl Bear cub from the Goblin camp, I somehow got it set on hostile and it joined the Goblins in attacking me. So, I knocked it out (1 hit point), and finished off the Goblins. I was curious whether it would later appear at my camp. Happily, it did!
So, I tried the same trick with Minthara. I killed the other two Goblin leaders, then reduced her to one hitpoint and left her alive. This was enough to complete the Save the Grove and Teiflings quests, even though she technically had not died.
Now, I'm at Moonrise, and no Minthara. I would like to suggest that Minthara actually has to die to complete the quests above or, if she's disabled but alive, that the player be able to save her at Moonrise (preferred option) and still complete the Act I quests.