Who said they're there to be neutral? They're the aggrieved parties... but that doesn't mean they shouldn't be informed of what's going on. How is it right to withhold this information from them?
In my view, it's less that they shouldn't be informed of what's going on, and more, if they're the first adults in the know (over, say, a trained psychologist or any neutral party), they're going to get the impression that they have precedence in deciding what happens to O&K in some respect. Like. During the whole table-kun debacle, Hitomi made her point that we ought not have the right to presume judgement, or however the quote was in its exactness. A set of adults, seeing a bunch of kids trying to run a semi-feudal semi-nation-state-alliance system, are not going to be
less likely to come to that conclusion, I think. But, with the way things are heading with the grand plans for saving mecugakind, for awhile, Sabrina and co.
are going to run into times where they need to be judgement, because other meguca won't be willing to listen to the reason of non-magical law. (To say nothing of the number of different flavors of local laws that we'll inevitably run up against, in time.) If we have at least part of a decided system, preferably formed with consultation with a neutral and trained party, in place about "this is how we deal with grievances of non-meguca toward meguca, this is how punishment works, this is where it's flexible,
this is where it's not", then we can hold that authority here, and keep our aggrieved Mikis and Shizukis from thinking that, as adults,
they have the right to pass judgement where we do not. And frankly, I think they have too high a chance of mucking things up, if they do that.*
*
Not saying we should be unilateral; this is my overly complex way of saying: Adding more uncontrolled moving parts (in this case, adults) into a precariously balanced system, is something that should only be done with the greatest of caution, and only at which point we're sure the benefits outweigh the risks. I don't think we have that surety yet.
they seem to be reasonable adults who will most likely be far less biased
We've interacted with them for all of like, an hour. Or- a fraction of a day. At most. That's not enough time to get to know how reasonable some adults are, in my opinion- Sayaka's turned out okay so I don't think they're secretly hiding being competitors for Bad People or anything, but you don't get a sense of how folks are gonna react to big situations in that little time.
Depends on what the repercussions are going to be. If they're going to flip out about their daughter having turned into an abomination unto nature and child soldier vigilante monster hunter and not want her to be involved with us anymore--which would hardly be an unlikely or unusual response for a parent to have to such a revelation--then telling them could do a lot of damage to her, us, the team and the city.
Also, this, too, yeah.
Homura and Sayaka (and us) will have less decision power about the matter
I do not think this point is going to make Homura more likely to want to bring the Mikis or anyone else into anything regarding Oriko and Kirika, ever. Homura will act unilaterally about them regardless of who holds the "decision power", if she finds them to be a threat anyway- and her going against that "decision power" if it comes to that would make it harder for her, to live in the world after Walpurgisnacht. Having known power over how they're not dead yet, is probably at least a little of the reason they're
not dead yet.
In accounting with all that:
[X] How does Mami personally feel about Oriko and Kirika? Listen to her.
[X] Telepathy Masami & Hiroko re: Ono, Kuroki.
[X] Science session: vote in abeyance.
-[X] Try making a grief-tesseract and toying with it.
-[X] Deploy Cardcaptor Sayaka jokes.
[X] Discuss O&K with Sayaka and Homura:
-[X] Respect their feelings; err on the side of caution. Stay positive.
-[X] Ask what they think should happen with O&K.
-[X] Bring up the long term viability of the house arrest:
--[X] Restitution: Financial reimbursement? Continuing using Oriko to help others?
--[X] Complications: Mental effects of confinement; Kirika's family.
--[X] Alternatives: Relocation, surveillance, restricted freedom with chaperones/tracking devices?
--[X] Justice?: Magical girls don't have laws, police, or courts. If your alliance grows... should that change? How? You don't want to unilaterally make decrees.
---[x] How to minimize risks while being good people?