Im affraid you cant take the groove for tieflings, they allways leave.
Even if thing goes wrong and you are forced to murder all druids (for several possible reasons), they simply go to Baldur's Gate after goblin problem is solved.
You can always save the Tieflings and when they are gone after the party, go back to the grove and wipe out the druids.
Don't you have to save Halsin and kill the goblin bosses to do that, though?
You do ... but that does not matter, players find something Larian didnt think about.
They joined Tieflins, saved the groove (Halsins survival is irellevant in this scenario), then celebrated ... and next day simply walked in the groove and start killing ... no story included, no EA consequences present (even if you have Halsin in camp, he wont react) ... no reasons to do that, but you certainly can.