I don't think asking more questions about her shades is going to work, she's made it pretty clear she won't answer those questions, even threatening to attack us if we keep up on those lines. Saying that we need to try to break through her shell is all well and good, but how many people will let a complete stranger do that in a single conversation? This is the kind of personal situation that we would need either metaknowledge, or several weeks of time to get to know her, or both to get through to her.
On the other hand, attacking her with civilians around, especially when we know so little about her abilities is reckless.
If she is going to be blunt, then we need to get blunt.
[] Are you interested in curing witches or not?
-[] If yes, tentatively agree to work together for that one day and adjourn to update your friends on the situation.
-[] If not, 'I think we've talked enough for the day, and I need to confer with the rest of my team anyway. You have permission to remain in Mitakihara until your return flight, but if you reap any souls I will fucking shatter you. Feel free to contact me again if you actually want to do business.'
I know some people are tempted to attack the evil right in front of us, but its a bad strategic decision. If we don't come to an agreement with her, I say we add her to list of post-Walpurgisnatch things we need to take care of next to the Iowa group.
I have a suggestion.
Get Homura to time stop and come over. Get civilians out of the area, fill body control range around Riona's body with explosives. Detonate the moment we exit time stop.
Rionna's soul sense might notice if people start vanishing around us, which could warn her enough to defend against an alpha strike, meguca reflexes are bullshit, especially top tier ones.