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—
"Do you think they wouldn't notice? I suppose the gate's far enough and we can release her early enough that she could detour and make it look like she came from the right place..."
"Yeah. We'd be cutting it a little close but she travels invisible anyway."
"Fair enough. We can move her to another room and interrogate her remotely so we can talk while we do it."
So they move her somewhere in the mortal world after Promise has healed her and Yellow has cleaned her ears, then set their conversation rig up.
"Was the list you gave me in order of relevance? If so, ascending or descending?"
"Whenever you hear the word 'mom,' you are to do your sincere best to not let anyone suspect anything unusual happened on your trip." He looks at Promise, releasing the button that relays his voice to the other room so the fairy won't hear them. "Good enough?"
"Yeah, I know, it's just good to have it set up in advance for the more common case. Would 'divert suspicion' work as an extra order itself or would I need qualifiers for it to do its job?"
"I don't like 'divert', it'd tend to wind up with her having the option to accuse someone else."
"Minimise? Kind of a mouthful. Reduce? Evade? Avoid? Depending on how she's ordered I could just say 'lie'..."
"Don't say 'lie', she can say any false thing she wants if she gets 'lie', it wouldn't even have to respond to the question. We could say 'I belay that', and have her revert to our orders in lieu of following theirs."
He presses the button: "What's the phrasing of the order about anything having happened during the trip likely to be?"
"Do your sincere best to cooperate with me and obey what you believe to be the spirit of all orders I have given or may give you according to your sincere best up-to-date model of me and what you expect me to have meant." He tries again, seeing just how far that order can get him: "Can you give me a few examples of phrasings of the order to tell about anything that may have happened on your trip?"