I've done some testing with this, trying to see if you can get something to happen by knocking people out like getting both Halsin and Minthara, but so far I've not been able to get Dammon or anyone else to appear. The most I've had happen is Halsin showing up a couple of long rests after the grove was raided to try and kill me, which is better then him just vanishing off the face of Faerun, I guess.
I've found that the triggers for who shows up to be somewhat buggy. One playthough I helped the grove and convinced Rolan to stay with the refugees, only for most of the Tieflings to show up at the last light except Rolan and the others that move with him. it's why the "leave no one behind" achievement has been so tricky to get, no matter how well i think I'm doing, something always happens.
As far as i'm aware, if you go to the mountain pass without stopping the rite of thorns, it will seal the grove and the Tieflings won't show up at the Last light, having apparently died on the road.