I want to gather some of the Rionna dialogue here for easier review., and then present a position. It seems we could negotiate from a "friendly unilateral" position for a while.
She is just that bad at negotiating that we have to play both sides for a bit. She is not able to transact the needed amount of information to gain trust.
Rionna is also terribly un-informed. She has no idea what we think, other than "don't like murder." Duh.
Helping people that you hate is an impossible thought for her right now. But we can break that.
#####
Let us be careful to remember what we have, and have yet to say to her.
You sigh as you sit down, Mami at your side. "I'm interested in working with you," you say. "There are... projects that require manipulations outside the scope of what I can do, but you might."
Be hard for me to not know. Why d'ye think I came here? It surely wasn't for your acquaintance."
"I want what everyone wants," Rionna says, and fixes an unblinking gaze on you. "I see people's Souls. I know what happens. I want what you have."
"Yerra, y' can't help me at all, can you? Y' can't exactly give me your magic, I doubt you're willing to give me your Soul, and I wager I'd have a fight on m' hands if I tried to take it."
"I..." you wet your lips. "I appreciate you being frank and upfront. I'll try and do the same - and, uh, if I'm monologuing, feel free to interrupt me at anytime?"
Rionna shrugs, eyeing you and uncapping her flask to take another swig.
"I want to create a better world," you say quietly, looking down at your hands. "Your previous actions might have been necessary at the time. I don't..."
Second chances.
"You don't judge me," Rionna says. "Y' have no right to."
"I... look," you say, grimacing. "I want magical girls to work together. I know things right now aren't ideal, but working together, with mutual support - I want there to be justice. Justice, protection, equity, accountability for magical girls so we don't all have to struggle."
"The thing is, I can cleanse Soul Gems and Grief Seeds," you say. "I can even empty Grief Seeds out to become Clear Seeds, like this."
"I... I'm sorry," you say. "I don't know you."
"Mm," Rionna says. "And you do not have trust in me. Understandable. You want me to not do what I do."
"Well..." you say.
"No," Rionna says. "I cannot."
"Cannot, or will not?" you ask, bristling. "Because I've got some ideas-"
"Cannot and will not," Rionna says. She drains her flask, tipping her head back for the last drops, and sets it aside. "You do not trust me. I don't trust you."
But here's how it is: You don't tell me how to do things, and I don't finish a fight neither of us want."
"I can't tell you how to act," you say, and meet her eyes squarely. "But whatever it is you're dealing with back home in Edinburgh, I can help you with it."
"You can't help me," Rionna says, shaking her head. "Go back to your girl, go back to your friends. Y' don't want this fight, neither do I."
"You're right. I don't want this fight, and neither do you,"
"So let's say that aside, and agree that I'm not trying to provoke you into a fight - and that you're not trying to provoke me into a fight."
"Mm," Rionna says, non-committal.
"But," you say. "I'm starting to think I want to help you with this. Why can't I?"
"D'you wanna give me your Soul?" Rionna says, with a rough snort. "No? You can't help me."
"Is that my only option?" you ask, raising your eyebrows. "Not to put too fine a point on it, my power has turned out to be a lot more versatile than my Wish would imply. I don't know what your problem is, but you don't know what I can do, either."
Something flickers across Rionna's face. Discomfort, maybe.
"Yes," she says.
"You know, I wouldn't put any preconditions on trying to help," you say, stomach flipping. It's not quite a lie. You don't like her or what she's done at all, but... Everything can be fixed. You have to at least try.
"Liar," Rionna says, completely without rancor. "Y' despise what I've done, and you want me to stop."
#####
We could say we aren't willing to murder her to make her stop. But we could work hard, and get to the same result peaceably.
She's "expecting things backwards," because she is defensive, asocial and selfish. These are adaptations to her issues as a magical girl. She honestly believes she could only solve her problem with her own power. Other people are as bad as she is, or worse. By abandoning all trust in other humans, she is trapped. We can't just leave her in her fate, for several good reasons. Therefore, I think changing her fate is worth our time
Is there any reason we cannot proceed forward without questioning her about her secret? That is the point. We could break into this defense two ways.
##### One, push proof of our position into her. Our position is that we DO help witches, and it is nothing new.
##### The other way would be to take her choice way, by raiding her secret, and (if correct) de--witching the sister without her consent.
I think option one is less controversial.
Or... Let's say the reason Rionna s trying to get Grief powers is to live freely? Or perhaps to become a mortal again? When she says "what everyone wants," the idea of saving witches is not a great match, only by limiting 'everyone' to the veteran population does this make sense.
What have we tried so far?
Telling her we have really good thoughts, and do good things.
We have proof of deeds, but we have yet to present it.
But hasn't seen moral proof, and she doesn't care to look for it. Baka.
We showed her a really good trick with our powers.
We tried falling for her self-judgment trap.
Further, Rionna has seemingly condemned herself already. She is the worst of the lot, in her own view. How do we get her to accept that we don't attack her for that? Become more extreme than her. We ARE in the right position. We freely try to
aid witches. We imprisoned Anri, but we aren't trying to punish her - in fact, if Anri is in the same situation as Rionna, we have made the best choices possible. Even from Rionna's position. We are trying to protect witches from Kyuubey, by keeping them on this side of the warp gate. We cure evil with things other than evil.
Rionna still doesn't want to trust? And maybe never will?
More than that, Rionna wants radical independence? But that's OK, we WANT to give her our ability, not our soul. It doesn't matter WHO SHE IS, AT ALL. We want to make synthetic Grief cleansers, and we want other girls to be able to make them. We want witches to be turned back into girls everywhere. This should match perfectly with Rionna's interest. There Is Always A Catch, but this is normal - hard work is required, shortcuts - like stealing our soul - aren't acceptable. If she wants to never be a witch then all she has to do is work harder with us, be a proper apprentice, and get the power that way.
By changing the world, her criminal acts will no longer be justified. We will in fact want her to be better, but we aren't crazy. She isn't courageous, and she is stuck in her past, perhaps. We can make the first change, and it isn't a trade. No Magical Girls died by our acts in Sendai. That day Sabrina is aiming for will come, even without her. When Magical Girls all over Great Brittan do not need to hunt, there will be no more territory in Edinburgh. And there will be no more standing armies, or a need for defending her city. As long as she understands, that we will have a requirement upon completion. She has time to consider, and time to change.
I will iterate my vote later as time permits, or copy what you think applies and do it your own way.