Slightly updating my vote.
[X] Hijiri's still a threat, and Niko's angry and not treating her fairly.
[X] Give the Kannas space if they mind you being there, but stay within sight and listen in.
-[X] Grief Fog ready, if case it's needed.
[X] If the conversation progresses well, let it play out.
-[X] Else, interfere. Mediate. Try to keep the Kannas calm.
-[X] Appeal to their rationality, help them make their cases, and understand each other's point of view.
[X] If the conversation seems hopeless, bring up the current crisis. Yuuri.
-[X] Get rid of the Memory Device and ask to be put in contact with Yuuri.
Make no mistake, we're gonna draw some heat and incense the Kannas, unless we just sit this one out.
Because dangers, that's likely not an option.
Well... If things go well without us, I want the option there to sit this out.
If not, I don't think bringing up 'we're also a wishgirl' will help until Hijiri gets a chance to calm down. She's focused on something else right now, and it's not the time to either drop some cryptic hints, nor to go explain our backstory. Focus is on Kannas.
And if we do need to interfere, well, draw some heat, try to get the girls calmer at each other, at least. If they get angry at us for interfering? At least their anger is dissipating a bit, and they won't be as mad at each other. But still, it's not very likely they'll listen to us if we try to drop some lines and step out (as if we had the 'right', or 'knew better'...).
But we can try to keep things calm, and offer our understanding of the situation, as long as we're only supporting the Kannas in their discussion.
And if things don't work out, distraction! Yuuri is still out there, let's go!
(What I'm not really addressing is the rest of the Pleiades...)
EDIT: Maybe ask Mami to help?