Well if Truck-kun ends up being in this story, I imagine he wouldn't have much of an issue. It would be just another job well done for him, while he keeps trucking on.
For the record, I never personally saw Miki as being especially brainy amongst the Charas - just more creative and artistically inclined. Her introductory sequence highlighted her apparent flaws as having a potential for mood-swings and a tendency towards emo-ness.
I expect Miki to start wallowing in emo and questioning her own self-worth and utility in the aftermath of Amu's rejection of her grand plan, before someone with an actual brain bothers to point out that advertising oneself as a human shield is not how you push up your market value.
Well, I'm pretty sure the exaltation shard already got trucked! Transit in progress, at least it's got company.
More seriously, I don't find emo to be compelling writing, and isn't Amu the goth girl in the room? Not that she's doing a great job of it. 😅 But Miki... you're kinda right. She isn't the most cheerful person, is she? I'm not sure I'd go as far as to assume she'd lock herself in her room and sob into the pillow, though.
It's, hmm.
Midori and Tsumugu were assuming that their hands-off parenting style
worked. But even in canon it meant they missed an entire magical girl story of sorts, complete with Amu having chunks of concrete bouncing off her... even if it all worked out, there's probably some timeline in which that didn't. Moreover there's all the ethical issues with– everything, really, though less Amu than
Utau, Ikuto, Easter as a whole, Nikaidou, Lulu, Hikaru...
They're going to need a firmer hand.
Left to her own devices, Miki probably would flounder for a few months and then figure herself out. If she got very unlucky, that might be enough time for something bad to happen. Fortunately, and
unlike every other situation like this, Miki actually has parents. Parents who are motivated to keep a closer eye on her.
This vote was– I don't think it's much of a spoiler– a vote for Amu's immediate reaction. Not for Midori's, because there's zero chance that any sensible adult would let a thought pattern like Miki's go uncorrected.
Let alone her mother.