I think you may be talking past eachother slightly, in that it looks like literally nobody is arguing that Rionna has a right to go free right now.
Also, that said, I would like to not use the logic of "it's not wrong for meguca to go around righting wrongs and stopping evil" or "most protagonists would have already just jumped to shooting" to avoid examining how we go about it, or to avoid conducting ourselves to as a high a standard in anticipation of the legal system we want to create as possible. Because it's
that self examination and standard of excellence that, both in the public eye and generally, is going to be what separates what we're doing here from what Rionna started off doing with her "let's go around ganking and nomming the powers of murderous evil griefgucas" business. I mean, yes, we're not malicious, but burden of proof for that will be on us, publicly.
There are of course various other things that separate our conduct from Riona's, but several of the thoughts I have on them would probably verge on reviving the morality debate that we're not to have here, so I'll leave it at my more immediate practical concerns.
(Regarding legal systems and our plans for them generally, not the argument about them
While yes, a system that would free Rionna at this juncture would not be a legal system that we'd consider a legitimate system, given the weight of evidence we have the capacity to collect and the knowledge given to us by Firn, legal representation and the right to it is also relevant in the context of securing just treatment. (Whatever that eventually shakes out to being.) It's my belief that we do clearly have standards of acceptable treatment (as moderated by safety concerns), evidenced by how we deal with Anri and Oriko and Kirika, and upon construction of our hypothetical legal system, I believe there
should be methods in place such that magical girls subject to that system can be guaranteed that acceptable treatment.
It won't just be the Mitakihara band running individual cases, when we get a larger system up and running; it needs to be able to hold up while we're engaged with other pursuits. (It also probably is going to require an expert in multicultural messes, probably.) Because however much we trust Sabrina and co. to hold to whatever standards we decide upon as acceptable treatment, it'll be important for the general populace of meguca to be able to trust them too, and that means having a system that includes representation for the accused and guilty to make sure we're not just... dictating. Or basement-canning-ing. This is one of the cases where the appearance of what we do
is gonna matter a lot, for the reputability of our system. As well, no matter how much we trust Sabrina and co., unless literally all the lawkeepers are Sayaka + like Yuki for the cases where Sayaka would be a conflict of interest, there's going to be instances where our legal system is going to be being implemented without our direct supervision. In those cases particularly, representation (and a solid chain of evidence and paperwork, probably) is going to be important.
So like, while representation in the sense of "determine whether or not you are guilty of this specific thing which we by all appearances know you are guilty of" may not be of much use, Rionna's (and future megucas')
rights to that representation- including representation that is there to make sure we
do gather that evidence we are capable of getting, again, so that there is
no room for us to be appearing to condemn someone for personal reasons without that evidence- is still something that she's kinda got to get, as soon as we have a system running, if we do in fact intend not to just dictate it all manually. If we know she's guilty, then the legal system we intend to construct better be robust enough to determine that by itself, and then she should be subject to whatever standards we've put together within that. At that point, legal representation becomes relevant for the whole "acceptable treatment" thing, which does in fact also include making sure we're
not just shoving a conscious Rionna into a no-activities box,
or keeping her in stasis for our personal convenience rather than her preference or practical security concerns.
... I feel like I forgot something, but that's most of it.
... also, slightly ninja'd.