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—
How about Eyak? There is only one person alive who currently speaks it, apparently. His employee also helpfully suggested Osage as a recently extinct language (as of last year), so Mortal asked them to figure out how to say 'wave' in that language, too, under the reasoning that enough people kinda more-or-less speak Latin but that one is really unknown other than an apparently doomed project to keep it alive with a very incomplete dictionary.
"It seems to be related to usage somehow, but I'm not sure how to easily distinguish between hypotheses..." He shakes his head quickly. "And anyway this isn't very relevant right now, I'm sure amongst plain speak recognised languages we can find ones with shorter words and sentences and symbols for the relevant orders if we need to."
"It can often be, some sounds are just naturally easier to cram together and say faster in a human mouth than others, and some words are just shorter in some languages than others, too."
"The stamps should arrive soon, anyway," he says, and checks the feed for Verve's activity.
So they can wait a few more minutes until Mortal receives a text and a motorcycle pulls up and he goes to the door and the person who arrived does not go to the door and he meets them and gets a bag with stamps and returns inside.
"I have stamps! I'm also a bit peckish, want food?"
He leaves the bag on the table and goes fetch some fruits for Promise and makes himself a sandwich, then goes back to the operations room—for lack of a better word—to feed her.
How about using separate pieces of papers with letters to form words?
"Assembling the papers looked like something you could do incorrectly under stress anyway, but the stamp's a pity, yes."