Firstly, is there a way I can rescue Halsin and save the grove without aggroing the entire goblin hideout?
Yes there is! (I'd put a spoiler tag here, but the whole thread is a spoiler.)
Talk your way into the goblin camp, head straight for Minthara and tell her where the druid grove is. Take a long rest, then go to the grove and talk to Zevlor. When Minthara shows up, betray her. After the battle, Halsin just shows up at the grove, having saved himself.
That's the fastest way I know to save Halsin. Ragzlin and Gut won't be dead and you'll need to find another way to the underdark if you don't want to face a hostile goblin camp head-on. I think the easiest path is sneaking through the spider lair in turn based mode and Feather Falling down the chasm.
The underdark is scary because everything wants to push you into the abyss. Both merchants in the mushroom circle have some good loot to sell, including the Saphire Spark which is a must-buy if any of your casters know Magic Missile.
Edit :
Are there no "area zones" that remain neutral? For example, if I kill everyone outside, are everyone inside automatically hostile? What if I only kill the sleeping Gobbos upstairs outside, are the revelers downstairs also all hostile?
I'm not sure what you mean by the first question, but the answer to the other two is no and no. Only killing Ragzlin will trigger the wrath of every goblin in the camp. Otherwise, they need to catch you in the act (killing or otherwise) to be angry at you. (The weird exception being that the goblins outside will immediately suspect your party and turn hostile if you poison the booze.)
