When you reenter the Druid Grove the druids should have emerged from the stronghold and left the stone door open. There's a big fight with maybe 15-20 druids that's IIRC separate from the smaller fight that kills Khaga. I think it only triggers after you kill the tieflings outdoors.
Don't forget to also kill the frightened tieflings in the safe room behind the other stone door, near the markers/training area.
You'll be able to talk to Minthara after that's all done. Be sure to save regularly, there's no opportunity to once the final dialog with her starts.
