As for letting the kids talk it out...I don't support it for one simple reason.
They all have problems. Mami is still holding onto the witch bomb, even if we did test Lobotomy on her. She won't be hard enough against Sayaka. More importantly, the baggage she carries with her reputation as a "Senior Magical Girl", won't allow her the correct advice.
Madoka simply doesn't get understand yet, and from Sayaka's pov, that means her opinion is irrelevant.
Homura...the whole problem came from her inability to talk so that doesn't even matter.
We did this before. Ayin did this before. Ignoring someone until it was too late. Not just once...but many many times. Besides.
With Nothing There lurking around, I would rather talk her down now and not have to face her again as a piece of NT's shell.
FWIW, I'm not suggesting we never broach these topics with any of them
ever. Just not right
now in this specific conversation. You're right that they all have problems but most of these problems exist even if the speaker is X, if occasionally in a different form.
None of them have the full picture and
none of them can handle having the facts just dumped on them. Homura is the most knowledgeable but even she can be driven to despair by being told how her efforts are what keep increasing Madoka's karmic potential with every loop. And telling the others about Witches or even "Soul Gems literally are your soul" are things Homura has tried with
atrocious outcomes. So regardless of whether the speaker is X or Mami they can't accurately impress the situation upon the others whether it's because Mami
doesn't know things or because X
can't shouldn't say things. We can try anyway but run the risk of sounding like a trope to the others - Sayaka already seems keen to think of us as the jaded, cynical vet and since we can't go "I'm not kidding, if you're not careful - physically and mentally - you could become a witch" we can't really impress the actual stakes on them to match our tone. Or worse, we become just an adult lecturing her/them about what to do despite only knowing them for a few days and without even being a Magical Girl even if we're Magic-adjacent-ish and she starts tuning us out and not taking us seriously.
I also think it's pretty unrealistic to think we can solve all their personal and group flaws in a single day or speech. I see it as more of a medium/long term project. If the goal is: "Get Sayaka to have a properly grounded view of being a Magical Girl (without being able to inform what that actually entails) and without taking a whack at her self-esteem (which would also be a huge problem because Grief) after she just had to trek through a barrier where hospitalized people (which, for all she knew included her crush) were being possessed by eldritch abominations and had to first come to grips that actually this may not be like her magical girl animes. Oh and also to resolve her issues with Homura." I feel like that's a complex project! Maybe not the Homura part, actually - I think that's comparatively a more straightforward issue.
So, with that in mind, I think the best thing we can do for her (and all of them, Homura and Kyoko included) is basically to look after them and provide them time, space, and opportunity beyond the endless fighting for Grief Seeds to stay alive. And to give advice and be a sounding board when needed. Eventually we need them all to get to a point where they
can handle the awful truths if only because
Kyubey would be able to weaponize them. If I genuinely thought Sayaka was a risk to go off soloing Witches right now I absolutely would not think it'd be okay to stand back and let them talk things out themselves, but I don't think she is. If anything, I think this outing should have demonstrated to her the dangers of Witches beyond the danger to herself and the benefits of working as a team. As long as that appears to be the case, I think we're okay. Because then we can keep speaking with her and trying to work on the rest. And right now I think Mami and Madoka can and will reinforce that if they talk amongst themselves. If somehow that conversation goes off the rails and we've got a code red where Sayaka explodes or sulks then of course then I think we should step in but that doesn't seem likely to me?
I mean, end of the day, like I said, I don't particularly have any strong issues with the write-in. I just wanted to offer an alternative.