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—
He writes the little star that's supposed to mean 'unfold your wings' on a piece of paper in front of her.
"Was a long shot," he sighs. "If I were to try to extrapolate from this, I'd expect it has something to do with the number or proportion of people who actually used and understood the language at some point in history."
"Well, it hinges on something... I wonder if simple ciphers work. Actually I wonder if dead languages do work, even. And if I have to even know what I'm saying."
"Hmmm... I'm gonna ask my employee to find the words 'wave' and 'hand' in Japanese and send them to me without telling me which is which then see if I can use one of them to order you." He emails something to that effect. And then he grabs two different ciphers. "I have an idea about a possible way to test the thing about number of users."
"A few ideas, actually. One is conlangs—that's invented languages -, the other is languages spoken by only a handful of people alive, and the other is using common and uncommon ciphers. More specifically, there is for example this one cipher invented for a game a few years ago and some people sort of speak it, but not, like, a countryful of them, and it's technically just English with the letters changed around. And then there's a cipher I can make up right now, which will still be just English with the letters changed around but in a way no one uses."
"...oh, erm. Mortals need to encode their speech because of lack of magic and we encode it in stable sounds and there are minimal bits of those sounds that get combined in different ways to form words. It's actually a bit more complicated than that because sometimes a single letter can represent more than one sound at the same time and even sometimes different sounds in different places, and it's pretty fascinating and I could tell you a lot about it if you want, but the only relevant part in all of that is that if plain speak works with the cipher lots of people know and not with the cipher no one does then that's pretty conclusive evidence about at least some parts of what it cares about when it comes to communicating with humans."
"Anyway! I got two words, one for 'hand' and one for 'wave' and I don't know which is which, can I try using one and the other? This one has the most practical relevance, I think."
"Okay, this is good, means it doesn't rely on my understanding of the words, or at least not exclusively, there might be languages and alphabets where writing these orders out is faster than sign language. Probably not faster than stamping, though, so if that works then best possible thing." Beat. "I'd still want to figure these things out for the sake of science, though..."
"I was using 'science' here as a shorthand for 'the drive to figure out how things work and why.'"
"Okay, so, ciphers. This one was used by the game and bunches of people know it—it's called Al Bhed, if you're curious. Fyja," he tries to order her to wave, pronouncing it 'feae-jaeah' according to the guide he found online.
"Okay, cipher doesn't work, now..." He emails an employee and gets the word 'wave' in a conlang, a nearly extinct language, and Latin. He tries the conlang first.