If you kill Zevlor, report it back to Kagha, and then Long Rest (i strongly advice doing it OUTSIDE of the Grove ...
The Grove will get sealed ... Tieflings will be gone ... and all druids inside will become hostile.
There is (at least curently) no reward for this approach, and you cant then side with Goblins, bcs Minthara will be pissed that bcs of you she cant massacre the Grove as she wanted to.
So your only way then would be side with very depressed Halsin, but you can leave Goblin leaders alive, bcs he no longer cares if they die or not!
But it is possible!
Ah, I see. Not terrifically satisfying as a narrative direction—missing out on the Minthara plot branch is a bummer—but at least it is an option. Still, maybe just siding with Minthara delivers more of a payoff in every regard; the way things stand currently, Kagha doesn't offer squat for siding with her.
So as you keep pushing him, it makes sense that he become hostile ... but i agree there should be dialogue option to say "fine, have it your way ... coward" or something like that, there is really no reason for *dialogue choice* if you have only one.

But no, as Zevlor say there is no way they would ever leave the Grove unless Goblin leaders are dead ... so, no chance to lead them ... well, anywhere.

That's just the thing that gripes me... there should be an option to
trick him, to make him believe that the goblins are dealt with and it's safe for the tieflings to leave—only to find that the goblins are alive and well and waiting for them. This could be very interesting especially if you have Wyll in your party, or even back in the camp.
By the way, thanks all for your insights, much appreciated!