I feel as though the most obvious way to allow minthara to be recruited without genocide would be to allow *nonlethal* attacks count as *stopping* the goblin leaders. Or maybe convince her to look for the grove in a completely different direction. Which then she leaves. You kill the other 2. And bam recruit her in act 2 after she failed*. It would be cool to get her in any of my playthroughs but if its between her and karlach. Im standing by Mama K all the way. (Yes i could play as karlach but i much prefer playing as tavs or durge atm)
