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 can talk to an empty room for all I care. We can have him tongueless and handless if he's still too dangerous and that would still be a better deal for him than being a sparrow. The important thing is that he's neutralised."
"...ah, yes, Blossom. Well, keeping them together in the mortal world isn't particularly worse than keeping them together as birds."
"Yeah. Not an insoluble problem but yeah, feeding them becomes a bit of a hassle."
"I suppose they'd probably prefer it. If you think they can be securely held in the mortal world and nobody will find them and get co-opted and let them go..."
"That would not be beyond my means but it would require a reasonable amount of supervision. Less if we don't care about feeding them personally, although, if they got the food and then fed each other would that work?"
"Not if it's mortal food. They can feed themselves, if it's fairy food."
"Then giving them fairy food is a bit less work than feeding them personally and a bit more work than just giving them mortal food and not caring about unsuspecting humans suddenly getting new vassals."
"It would be nearly as easy for Thorn to use an unsuspecting master he had access to as an unsuspecting person he had no vassal relationship with."
"I don't mean to give him access to said unsuspecting masters, I mean to completely automate food delivery so he wouldn't actually interact with anyone."
"Then maybe it doesn't matter," shrugs Promise, "but only if you can actually isolate them that well."
"Shouldn't be very hard, if a little bit expensive and annoying. Sparrowing them is still strictly easier."
"Well, they could still be kept as sparrows in the mortal world—unless that'd be dispelled, as well?"
"Okay, then yeah, not strictly easier. And I wanted to ask something and forgot." Button-press: "What's your kind magic, and how much sorcery do you know?"
"Evil McEvilperson, species: evildoer, powers: to do evil," she snorts disbelievingly.
"I'm sure it would be useful for other purposes if he had other purposes."
"It could be incredibly useful, especially if his cutting powers are as absolute as fairy immortality is. Is he a one-of-a-kind?"
"Then maybe I could find another one of his kind that isn't terrible and do science with them."