The event is basically something that Kokosando (one of the main dev, also artist of several students like Seia, Reisa, Kanna, etc) have been begging to be added to the game for the past three years, citing that he really wants to turn all the students into magical girls (and little girls, which is weird thing to say but whatever, guy's really into it).
The writer of the event is a brand new writer who literally started his writing career with Blue Archive. Reisa was picked because she has that hero-like personality + "being magical girl is something she would do", and Suzumi ended up also getting the magical girl treatment since she's on the same club and it would be fitting for her to essentially getting dragged into it and become magical girl as well.
The new writer would spent his waking hour learning about Suzumi from her in-game lines and it apparently affected him badly enough that the devs saw him in real life (the workplace, pretty much) muttering about “In this situation, Suzumi would say this to me,” or “In this situation, she would react this way toward me,” and the devs are thinking "maybe we should sent him to the hospital or some shit, he's losing it".
Aside of that, they planned several bad ends to the event but decided to scrap it for a more happier good end, and the exaggerated story bit where Shimiko speed-reading manga and magical girl material to become magical girl villain is based on devs using the same thing to learn about magical girls themselves. (albeit without the speed reading bit)
And speaking about Shimiko, the design we got on the magical girl event? That's the tamest one they got. Whatever designs they have and scrapped are forever locked inside Nexon's office and only by working there that you can see them.