outright chose to attack her, I was able to kill her there.
Doesnt seem broken to me, you were simply lucky on dices ... or find more effective way to kill than Larian expected.

At first I thought it might just have been a decoy
Nah
but when I got to her actual chamber, where the "real" fight with her was supposed to happen I was unable to see her NPC and I was able to free Mayrina without triggering any retaliatory attack from Auntie Ethel whatsoever
Thats bcs she is dead.

That is not broken, that is consequences.

But with a debilitating side effect that I could not walk through the second Illusory door that led to the alternate portal to the Underdark.
I was never able to walk through those. O_o