Your initial proposal was flawed to begin with.
My initial proposal is how real, functioning, long-lasting communal societies
actually work. Like, in real life. With real humans who are subject to real human nature.
I'm not talking about hanging criminals. You may note that we haven't hung the Iowa group, we're holding them until we can find someone to talk to them and help them chill the fuck out. Mature societies have "someone to talk to them" ready to go. They even frequently talk to people before they do any murders at all. Communal problems require communal solutions.
I never said that your all-hating mind control meguca would ever sit around doing nothing. It just doesn't matter, because I'm not interested in a "yeah but this countermeasure can be countered by that" VS debate. The weeds are uninteresting, irrelevant, and need not be gotten into. Assume that your supervillain is doing whatever SB Optimal stuff makes you happy. Likewise assume that everyone else isn't stupid. Remember how Iowa went? We had Mika call us to say "hey, this thing is weird," we said "yeah, that thing sounds ominous and deserves further investigation" and immediately told our closest allies about the ominous thing, further investigation revealed that the thing was indeed super ominous, and we had a bunch of our friends ready to go start a fight in about ten minutes. Iowa wasn't sitting on their ass doing nothing for that time, they were gathering intel in a super stealthy way that only a group containing someone very similar to Mika would've ever thought to look into.
The Iowa response was not representative of what we're aiming for. For one, our community is still small and inexperienced enough that having someone who can recognize the warning signs of a particular kind of weird magic is lucky instead of expected. For another, the girls in Mandalay couldn't directly call for help themselves. For another, they couldn't indirectly call for help by virtue of having friends in Maymyo who might notice that they're not returning their calls. For another, there was only a handful of people able and willing to answer any call for help that they did manage, due to a number of obstacles to both "able" and "willing" that we will be addressing in the coming years.
But, most importantly, Iowa came from outside us, and the only opportunity we had to intervene was well into their murdering careers. These girls didn't just decide on their own that it would make intuitive sense if the world was full of people all competing over the same limited resources and the only way to thrive is to compete better than everyone else so you can take their resources. They were raised in a society that encouraged that worldview, and then found themselves in an extreme environment that strongly rewarded acting that way and lethally punished not acting that way.
A Parró contracting ten years from now will still be raised in the same society, but upon contracting will enter a magical environment in which far more interconnected meguca are free of any scarcity except what they choose for themselves, with real adult magical girls all the way in their
mid-twenties to help her get acclimated. And then, because she's Parró and she learned the lessons of her upbringing well, she'll recognize that conquering other magical girls is a non-starter (even our Parró sees that attacking a powerful meguca's friends is a bad idea, she just doesn't realize that's scalable) but maybe she could find some muggles to rule as a god-queen, and someone will have to non-euphemistically discourage her from that.
If Sabrina has anything to say about it, a Parró in a hundred years won't even have the upbringing.