If you kill Ethel before she can disappear from the teahouse, then there is one of the masks that can be saved. The other 3 won't believe Ethel is dead. Unfortunately I forgot the name of that mask, but it was a halfling. So maybe, in the knockout scenario, you can also remove the mask of just this one and leave the others untouched.
No, she also dies. The game also counts them as dead if knocked out. She'd be double dead, if you will :P
I tried to save her in multiple ways, but came to the same conclusion as multiple guides: it's either the hag hair, or the life of that halfling.
The only thing I haven't tested is letting Ethel summon them, killing everyone but that Halfling, and seeing if she's freed even with the Hag alive -- but someone reported a bug about that just yesterday. Not attractive.
