I am Red Team. All your votes are belong to me.
— after all, as you remember, my avatar was always red. Do̢ ́n͢o̷t̛ do͘ubt͜ ͜the ̷tru̡t͝h ̷o̢f̨ t͟ḩ̛̛is̛͝ ̵͟m̶͘͝em͠òr̴͏͞y̵̡̕.́.̢̨
[x] Have Maiko start with whatever she believes is the most relevant to the upcoming meeting. She should have a handle on what information best supports your objectives.
[x] Ask Maiko to tag along at a distance, like usual.
[x] Ask Koharu if she has an interest in accompanying you (openly) to a hopefully-rewarding diplomatic meeting.
[X] With confidence and power — try to keep your demeanour similar to yesterday's, but make sure to subtly or not-so-subtly hint that your group has access to numerous resources and wildly different abilities. Try to keep it natural, don't go out of your way to show off or brag.
Yyyyeah, given the way we've behaved, nobody who's actually heard about us will believe that except for people looking for an excuse not to go anyway... and our sheer power is enough of an excuse for those people. Plus, it'd wear on Yui's mental state, I think, having to pretend to have killed someone.
Which is why I propose to weaponize rumours, not a full-blown act.
Magical girls don't have a centralized news network. How information about something like Momoko's death propagates to faraway places of Japan? I see four likely candidates: rumours passed on by neighbours or travelling magical girls, messenger magical girls, information-gathering powers, and Kyubey.
If we manage to somehow convince magical girls who never saw us that Nagamioka is under the control of Momoko's killers, at least some rumour-mongers and messengers would believe it, or at least tell someone else who may believe it. Starting it would be as easy as, for example, asking Rio Trio to tell some of their acquaintances, who they know aren't nearby, that there's a new group of magical girls in Nagamioka, who are ridiculously powerful and are probably the ones who killed its previous holders.
Girls with information powers and our immediate neighbours obviously wouldn't believe, but it would be their word against the word of someone else. Their word may be more credible, but it would still introduce
some uncertainty, which is good in itself.
Another point: Kyubey. It is nominally on our side — perhaps it could help us with this? It's probably the most universal, if not the most trustworthy, source of information for magical girls. If its on our side — and it being on a magical girl's side is probably rare enough that most puellae magi wouldn't consider it — that would give our rumours even more credibility.
In fact... why not discuss this with it? It's probably better at this kind of wide-scale manipulations that us. And yes, I'm aware of the incredibility of the suggestion of asking Kyubey for help in a PMMM quest.