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 was thinking inside the tree, in case my ability to hedge people out extends to people entering through gates."
"Oh, I was thinking just against a wall on the ground floor. Not too wide, just enough to walk through. Over there maybe." She outlines a box with fairylight lines.
She nods. "So you could make them and if one of them settles quickly enough we can verify."
"Yeah, I can make a whole stack right up against each other if the first doesn't go. Where do you want me to aim exactly on the other end?"
"Somewhere in the respective empty rooms of each place, far from the respective doors if possible."
"Do you think I should make a separate gate for each one? I don't have enough room to have them all up against walls where it won't be inconvenient to have them stand open and they won't interfere with each other, in here."
"As opposed to one gate into the whole complex. I can make a bunch of gates right on top of each other to destinations also right on top of each other, and then whichever settles first I have a functional gate where I wanted it; if I make them to separate destinations then for them all to be useful they need to not be right up against each other."
She blinks. "I'd—been under the impression that how long it took to settle was a somewhat deterministic function of origin and destination and making many gates from the same point to the same point would just mean they'd all settle at the same time."
"I'd expect it to be possible for small functionally-unimportant differences in location to affect settling."
"Well. The five safe houses are in different continents so we'll need at least five different gates, but they could all be on top of each other anyway and only one open at a time. Gate away."
Well then! "Okay, lemme deactivate security, don't wanna waste bullets on a twig." She sticks a head into the gate, says the phrase, then is back. "How long do you need to rescind my permission?"
"Hmm. Well, then you might want to make the gates outside? Also, it occurs to me that at least one gate must be open all the time if we want to be able to access our surveillance equipment from here."
Promise shuts the gate. "Travel gates outside, a little one for transmitting signal inside?"
"Works. Or, if we have a little one for transmitting signal, we can just keep the travel gates shut whenever we're not actively using them. They don't need to be open all the time anyway, do they? So they're not that much of a liability."