These two questions sort of share an answer. I wrote A Short History of Magic after writing a couple of novel-length Snape-survives-the-war noirish mysteries. For those stories I already knew I needed some actual dark magic mechanics for all the noirish shenanigans Snape was going to get up to. I had also noticed that in canon, Snape performs a few spells that are just different from any others we see. Long incantations apparently requiring eye contact and concentration, such as the countercurse he performs during one of Harry’s first Quidditch matches, and the healing spell he uses to save Draco from Sectumsempra. No one else in the books does incantations like those. Since Snape is a bit of a dark magic expert and he performs those incantations to fight dark magic, I felt that they could well be dark themselves. Incantations, magical songs or invocations, those are pretty common in folklore, unlike the single-word type spells that are common in Hogwarts. The other little glimpses of dark magic we get in the books, like Draco using a hand of glory, or the ritual used to revive Voldemort, are also close to the type of magic that turns up in folklore. It’s gross, messy, and has a fairly high cost. So I decided to have dark magic reflect how a lot of folklore traditions depict magic.

The first principle, the whole is the part and the part is the whole, was simple, it’s just very common in folklore, like in the effigy dolls that Snape mentions. Heck, anyone who’s torn up the picture of a hated ex has a little bit of that principle in mind.

The second principle, all energy must have a source, came because I really wanted dark magic to have a cost to it. Like Peter cutting off his hand to revive Voldemort, a large spell needs to have a large cost.

The third principle, the caster may affect the spell and the spell the caster… well, that principle is there because it is symbolically the theme of most of my stories. My longer works are essentially Snape coming to realize how his past actions and current maladaptations are slowly destroying him.

So, I had come up with these basic principles for my other stories, but I hadn’t fleshed out the history of dark magic or how it related to light magic, since that was beside the point for the longer novels. But I did want to develop that in its own story, since that is an important theme for me, how the dark and light are connected. On its own, it would just be a pretty dry essay or an info-dump, so this story is kind of a vehicle to be able to add some humor and character development at the same time. And, ok, I was also writing in reaction to some of the really weird retcons to the series, like that horrible tweet that wizards didn’t have plumbing until the 18th century, they’d just poop in a corner and vanish it. I mean… I guess that could be an explanation, or you could instead have magic evolve over time like literally every other human technology. Why not have magic be a bit harder in the past, something messier, costlier, that became more efficient and easy over time? So there wasn’t a vanish charm until much later. I think it just makes more sense. It also fits better with the basic worldbuilding that the wizarding world went completely secret centuries ago in a reaction to muggle persecution. The secrecy makes much more sense if wizards didn’t have things like apparition and easily-cast curses at the time.

Definitely Sarah. I’d already been writing Snape for my longer stories, so I was more used to his voice. Sarah is someone who only gets a couple of mentions in canon, so I had to flesh her out from almost nothing - in canon, she doesn’t even have a first name! Your question about flaws feeds into this perfectly - basically, all I had from canon are Sarah’s flaws. She’s the Ravenclaw who gets into trouble fooling around in the bushes during the Yule Ball, and she gets in trouble again trying to get past the age-restriction spells on the goblet of fire. So I decided that I would center her character around this trait of reckless curiosity. When she wants to know something, she will do what it takes to find out, no matter what the cost. It's not just a flaw, it's a perk as well. I wanted to give her a little affinity for the dark; she’s someone who indulges in pulpy occult thriller books (I had so much fun coming up with the titles and character names for those), so she’s had the curiosity building for a long time. Sarah is the one who gets a character arc in this story, Snape is mostly static. I had plenty of fun with his many flaws in my other works, but this story is more about how Sarah’s understanding develops through the story.