I wish this was a bit longer. Maybe not a full series, but long enough to see how Misaki disrupts the old status quo and the rippling effects of that disruption.
Also, I think the First Heroine was a bit off in her assessment. It's true that extras are drawn as faceless background scenery, because they're unimportant and the audience doesn't need to differentiate them or even acknowledge their presence beyond setting the scene in a crowd. By the same token, it's problematic if side characters are drawn as faceless background pieces too often, because they're not as important and not as distinct and they'll quickly get lost in such a position.
However, there's another type of character that can be safely drawn as a faceless bit of background: A main character. Main characters have so much narrative weight that fully drawing them in a scene will almost always result in them dominating that scene, so you can draw them as a background piece to place them in the scene and mask their narrative weight. Even in that diminished position, they still exert so much influence, and should be so sufficiently important to the readers, that they don't really fade away and there's no risk of them fading from their role.
Now, when Misaki came in she completely disrupted the old pace of the manga, changed the roles of previously important characters, and generally warped the narrative with her mere presence. Does that sound like a side character to you? Because it sounds like a new main character to me. One who might be occasionally drawn into the background so the artist can show her integrating with the school and her influence on events, even when she's not directly involved.
Judging by what Megumi was saying, and how the manga was depicted, it seems like it was starting to stagnate and, maybe, lose popularity. Misaki was obviously recruited, from a completely different manga and genre, to force a change of pace, either as the beginnings of a spin-off or the co-lead of a new arc.
Huh. That puts me in mind of an interesting idea for a sequel. Maybe Ms. First actually understands Misaki's position and considers her a threat, since her own position would be significantly diminished by a new lead and change in genre. So she's trying to force Misaki into a safer role. Perhaps she's even done the same to the other girls before and it's her fault that the manga started to stagnate.
And Misaki should have an impressive set of survival skills, seeing as she has been living and scavenging in a war area for some time... she prolly can use guns, jury-rig all sort of devices, fight with improvised weapons, make a campfire and cook food on it... but, so far, she hasn't had the chance to put any of these to use... that's a side of her that should definitely be explored...
In conclusion, leaving this manga as a oneshot is a crime. I demand a series! ヽ(͡◕ ͜ʖ ͡◕)ﾉ
So, what I'm hearing is Misaki needs to go Full Metal Panic?
