I see the angle of what you're saying, but I don't think it is a case of having it all because;
- Getting Auntie Ethel below 15hp and keeping her there until she gets her turn is incredibly hard as any class can easily overshot 15 dmg per hit.
- The risk of failing to obtain both Mayrina and the Power are quite high.
- You do not get to choose who does the talking as the dialogue is proximity based mid-combat.
- The Deception check is 20, the highest possible value and is supposed to be a false promise by all means of the word.
- You cannot apply any buffs for speech (such as Guidance or Shadowheart's Enhance Ability to give you an Advantage) because you are still in combat during this scene.
- You only have the Charisma bonus, Inspiration points and most of all, luck to succeed this roll.
And to make things worse, I managed to fail all 5 rolls consecutively as I was counting on my speech buffs to make it easier. Boy was I wrong when I remembered I was still in combat

Now that I think about it, this actually is the hardest speech check to pass in the game.
But if we were to fail in delivering a killing blow or continuing the fight, I'd rather have a [MELEE] roll check for that. Succeeding such a roll would deliver a killing blow or the fight continues to the death. And failing the roll, she simply runs away. That'd make some DnD immersion right there as far as I'm concerned.
But I definitely do want to kill her no matter what, so the fight will be the same-y for some individuals. I personally will never choose her power over killing her, as one attribute point is of less worth to me than a satisfying story outcome.