Leave the shadow-cursed lands for Baldur's Gate

Rest in that one odd camp outside of Baldur's Gate

Deal with your dream visitor's bullshit

Wake up the next morning and leave camp

STOP DON'T MOVE - don't go down into Rivington yet

Talk to Wyll

Get his conversation about returning to Baldur's Gate (if you haven't talked to him in a while, you might have to cycle through queued-up remarks)

Emerald Grove

Kanon - Meant to Die ™, streets saying he can be saved if you don't long rest before reaching the Grove but this was possibly only in EA

Nadira - Upon entering the grove, you can save her from the assassin

Arka - I believe she is Meant to Die ™ since even if she doesn't perish in battle, you'll find her unalived (possibly by her own hand) near the goblin camp

Mirkon - child you can save from the harpies, in harder difficulties they will target him first

Arabella - child you can save from Kagha

Shadow-cursed Lands (SCL)

Memnos - found trying to comfort Arka in the grove after Kanon's death, Meant to Die ™ upon reaching SCL

Asharak - training kids beside Wyll, Meant to Die ™ upon reaching SCL

Ikaron - helping Okta with the baggage, was in tiefling party, Meant to Die ™ upon reaching SCL

Multiple other corpses will be scattered around near the mountain pass route, they will not break your streak

Arabella - got separated from parents, can be rescued while wandering around Reithwin

Arabella's parents - Meant to Die ™ for Arabella to become who she is

Several other Tieflings such as the kids, Cerys, Alfira, Bex and Rolan will be in Last Light Inn. I believe you will have to ensure no deaths once Marcus attacks. Mol will be kidnapped as a canon event- this will not break your streak.

Rolan - drinking himself to death because his siblings got taken, will try to get to Reithwin area/Moonrise alone, can be rescued (and comforted)

Moonrise Towers

Cal, Lia, Lakrissa and Danis - can be rescued from Moonrise Towers. The guys will try to help the fight against the guards while the ladies and ironhand gnomes break the boat chain- their death may break your streak.

Zevlor - can be rescued from the Oubliette. If not rescued, Orin will serve you his corpse (based on other plays i've seen)- will most likely break your streak due to his immediate implied relevance (first tief you can approach in the grove).

Baldur's Gate

Some tiefs such as Zorru, Bex and Danis, and the kids will be in Rivington. The two tiefling queens will be in the Tavern. I believe there is no threat to them in these areas / no rescue needed.

Mol - in the Guild's hideout with her traders. Not sure if she can die in the battle (as collateral dmg) as I haven't played all difficulties yet, but her death may break your streak due to her relevance in final battle.

First of all, I assume you're doing your best to pass either the Intimidation or Deception check to convince Ethel to surrender Mayrina. Otherwise why are you even bothering with this?

Anyway, you want to reach this dialogue and then STOP.

Then, switch to the party member you kept out of combat with the hag.

This is important, as if they're entered into combat, they'll be confined to this cutscene along with everyone else (notice the speech bubbles next to the rest of the party). This is the trickiest part, as without a party limit extender mod, it's a big ask to go into the hag fight with just 3 characters (why I recommend taking her on a little later, but you can totally do it before getting into act 2).

If they're not in combat, upon swapping to them, you'll get the Reward Select screen for the hair, but everyone else will still be in the cutscene. If you want a guaranteed way to keep them out of the fight should it get dicey (with lots of Ethel clones everywhere taking up potential hiding space with their vision cones) you can try keeping the one party member outside the arena, at the top of the trap hill past the waterfall, and then just use Feather Fall (any source of it) to jump past all the traps down to the fight arena.

This is important (and a good reason to get her hp as low as possible, not just beneath the threshold of 30 needed to trigger her bargaining). You have to reduce her hp to 0 in one shot. That means the damage has to be dealt all at once (no pause for reactions like sneak attack or divine smite, you want to use those sort of moves directly from the hotbar rather than triggered on reaction). If you don't kill her in one shot, that character will be pulled into the cutscene as well, and your chance will pass.

If you succeed though, she will disappear and the cutscene will immediately proceed (I missed the chance to get a screenshot of the damage dealt, but it was over 25 so more than enough). There won't be a body to loot.

But, because she's been flagged as dead, all the hag's living prisoners can be freed (also shoutout to act 1 Minthara. Sorry you got the wrong impression of me with the whole killing the grove thing, this is also who I am. I contain multitudes).

Other than the mask bearers, I think I feel the sorriest for this guy, tormented by the worst visions of the future for "he's-an-elf-so-could-have-been-centuries" time. Unfortunately I killed all the mask bearers in my run, but if you can knock them out, and the game isn't bullshit about it, they should be safe to remove the masks without them dying.

One lice-ridden hair clump and suspicious potion later (thanks Astarion! Your role in the playthrough is now done!) and I have 20 strength with no ASIs. Perfect :D