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 want to ask a question but I'm not sure how to phrase it tactfully."
"And I decided I wanted to stay the way I was so I promised myself."
"We know what we are and what that means. Our names. The basic laws of the universe. Some fairies didn't start knowing about the Queen but I'm new enough to have done. How to move and forage and talk and write."
"She has an overwhelming advantage and has been very good at pressing it and she's old."
"Half-formed plans and ideas. How many continents, ballpark? And how large are continents here? And how on fairyland does she have such ridiculous reach?"
"Continents vary in size. This is a big one. She has a couple fairies who can travel particularly fast, I think some with passengers. And she's rumored to have sleeper agents far away. I would not expect to encounter the Queen by doing relatively ordinary science and court-shuffling if we went, oh, out past the salt sea, but I don't think I have ever heard of a location far enough that she couldn't find us if she wanted us - books suffer strictly more inconvenience in getting around than her vassals do."
"Hmmmm. And what kinds of things are likely to attract her attention? I have the impression she likes having the best—things, and while she can be excused for not having noticed that mortals are on the cusp of having much better things than she does eventually she will."
"I don't know. I think she may like having it generally known that she can do whatever she likes more than she likes doing anything in particular."
"I'm not sure how far I trust that. And I'm not sure how ordinary science I want to do. And I'm pretty sure the kinds of things I want to do will eventually bother her a lot even if I don't include the part where I'd like to replace her with someone who will do her job better."
"I stand by what I said. Long-term goals involve 'utopia,' and this might just include importing humans here because it's probably way less likely that fairyland will someday end."