I mostly like
@Vebyast 's take on the vote. I think the "drink the pain away" thing comes across as a bit too judgey/preachy and might get us snapped at though. Actually, all of that last segment can be improved.
[] Find out what her core goals are.
-[] Just ask her bluntly.
-[] Everyone has something that makes them keep going. Yours is obvious. Some people just refuse to die. But what about Riona?
-[] She obviously isn't flying around ganking grief spreaders out of the goodness of her heart. And it sounds like she can't relocate to work with you.
-[] So what's so important that she has to defend, and why is she collecting grief spreaders?
[] You need to know what she wants because you want to propose a trade, because that might help you work together even if you don't trust each other.
-[] Riona helps with Dewitching research and making the world a better place.
--[] You can promise that it is exactly what it sounds like, that you intend to do
good, and that you have enough power to refuse to make "hard decisions".
-[] You'll help Riona achieve her goals, just like you want for every magical girl.
--[] Can she promise that her goal makes the world a better place?
Even if it's just something that doesn't hurt others, but makes her happier. You have enough power to offer her means to her ends that won't be nearly as harsh.