This return to talking about how this whole thing is in fact an invasion has really reminded me how attached I am to the idea of actually trying to win this. Just without the genocide implied in the current plan.
I mean, realistically speaking, a genocide is basically unavoidable following the current plan. It's important to separate what a victory is for Zaiyuu from what a victory would be for the faction, because in these kinds of stories, the two rarely ever actually match up.
...This is also why magical girl villains tend to end up either dead or redeemed: Either they figure out their goals don't match up with their faction and leave, they find out that their goals don't match up when their bosses kill them, or they have their final battle with magical girls and get purified, which can either kill them outright or redeem them based on whatever the themes of the season are.
Anyways, Zaiyuu's goals have been stated to roughly be as follows:
1: Ensure the Estil have a future.
2: If possible, allow humans to maintain their current way of life.
3: Keep people he likes safe, but only if they want him to.
My main perspective on teaching magic is that it helps assist with goal #2, and while it does create a bit of disruption towards invasion plans, Zaiyuu's goal isn't necessarily global domination, so much as to ensure that the Estil can survive. ...Which admittedly, still requires them to at least have a significant presence on Earth or at least the moon, but doesn't necessarily require a successful invasion.
...Okay, look, I play Terra Invicta as Academy. Half the plot of the Academy route is disrupting the alien invaders just enough to get them to the negotiating table.
The other half is trying to stop the extremists from blowing up the planet, but that's just normal Terra Invicta gameplay.
(The true solution to the Estil surviving is almost definitely going to be the magical girls getting a Civilization Cultural Victory, because Estil culture itself is kind of insane and self-destructive. Even if they do take over Earth, what we've been shown of Estil outside of our group is that they casually blow up planets, so like. I give it 20 years before they doom their entire species by blowing up the Earth on a dare.)