The plan is refined and strengthened, contingencies are set to deal with various ways Thorn could've strengthened his defences, Mortal receives a very interesting email concerning one of their mother's contingency plans—namely that her assets have all been either frozen or transferred to Mortal themself, so Thorn doesn't have access to them -, and when Mortal and Promise judge there's nothing more to be gained from further planning they (eat dinner, sleep again, have breakfast, go over the plan once more when it's not completely fresh in their minds so they see if they come up with anything new, purchase Promise a mobile phone and a power generator to charge its battery in fairyland, eat lunch while Mortal teaches Promise how to use that, and) part ways.
On this side of the veil, Mortal gets to work. The first step: getting at least two safe houses, one for their HQ and the other for contact with the place near Thorn's court. They end up getting five, for redundancy's sake. The main HQ is near Seattle, the other four safe houses are in Greece, Russia, Japan, and Argentina. That, and getting the necessary existing equipment, is the easy part—you end up with contacts of the relevant sort when the bulk of your wealth comes from the kind of thing you can get with sorcery. The hard part is getting the various specific bits of technology that don't exist yet, including the the many types of trap and ammunition they'll need. Processing fairy voices with software turns out to be a dead end—apparently they're weird magical superpositions of sounds that make software go ?!?!?!?!?—but everything else, as agreed upon, can be made to spec nicely enough.
It'll take a couple of weeks beyond the one month for everything to be ready. Mortal hires someone who talks very fast.
And after the ball's going, there's not actually much for them to do with their time. They fret about details of the plan, order more redundant pieces of stuff (especially the to-spec stuff, not being mass-produced means they'd better have a lot of it to start with in case anything goes wrong), and have various antsy and anxious and calm and relaxed and terrified and panicked moods. A month is a long time...
At one point it occurs to Mortal that Promise might've decided to run away and not help, and then they'd never see her again and that would be terrible, and why would it be terrible anyway? It's just some fairy, fairies are evil, one must remember that. Even though she wasn't, of course, she was smart and resourceful and moral and ridiculously hot, and if they never see her again she'll never order them again and the tingly feelings won't ever happen again. Except what the heck, what are they even thinking? The answer, of course, is that they want to see Promise again. Why? To save their mother, of course. The only reason being ordered like that felt good was because Science. Of course. Of course.
The month passes—
"I have spent the past fifteen years thinking about all the things I'd want to run experiments on and figure out if I ever had a nice, willing, cooperative fairy to help me. I... had sort of been expecting to either have succeeded at figuring out how to hack fairyland or died trying before that happened, though."
"Which you were planning to implement while considering all fairies at least probably evil."
"...well, 'evil' is a shorthand. The way your vassal magic works creates some very, erm, unappealing incentives, from the point of view of a human. I don't expect we'd be any better as a species, did we have the same kinds of incentives."
"But you don't, so you don't assume that humans are all evil by default."
"No, but I... sort of assume all humans are... 'stupid' may be too strong a word, but they certainly lack a certain thing fairies tend to have, that they need to navigate this order-be-ordered situation. And even then saying 'I assume fairies are evil' or 'I assume humans are stupid' is a gross simplification that may convey the general tone of the thought but not the nuances."
"So what I really mean when I say this is I expect the average fairy I run into to care very little about an arbitrary mortal's—or even arbitrary other fairy's—wellbeing compared to the baseline I'm used to, and to in fact want and have the power to do things to said mortal-or-fairy that they would vastly disprefer."
"The dangerousness is in the fact that they have the power to cause harm. The evil is in the fact that they want to, or rather that the things they tend to want to do are harmful and they don't tend to care enough about causing harm that this stops them. Like I said, 'evil' is a simplification of a thing that in practice is different from 'dangerous' because it includes details of their values and motivations which need to be taken into account when dealing with them." They reach the third gate, another safe house very similar to the previous ones, this one in a mountainous area.
"It may not be the optimal way to think about it, given the emotional reaction it has actually caused." Pause. "And, er, for what it's worth I do know not all fairies are like that. It's just, for me a false negative is much costlier than a false positive." Back to fairyland, close gate, make way to the last place in the circuit.
Walk walk walk.
"I have mixed feelings about someone planning to take over Fairyland for the benefit of high-minded ideals while predisposed to even summarize all of its inhabitants with a term like 'evil', however that unpacks."
She scratches her head. "Okay, I see your point, but I also have empirically demonstrated I do care about individual fairies' wellbeing, on a gut level, no matter what twisted lies I tell myself to cope. I wouldn't be nearly as committed to this if I didn't. So I don't want to take over fairyland for the benefit of a high-minded ideal, I want to take over fairyland because the way it's currently run causes lots of suffering and suffering is bad. And anyway why would the way it unpacks not matter?"
"Because I don't think you'll unpack it every time you're presented with a decision."
"Okay, so, I want to start off by saying you're right, and the following is not meant to excuse or justify anything."