She was just given the name 'Hanako Shimura' by that nun, right? Maybe there was an original who fit the role of villainess better.

I think this, like a lot of fiction, uses exaggeration to convey feelings that are realistic but hard to flesh out in a concise manner.

I think the word that you (and others) are looking for is "Melodramatic". While that word is sometimes used disparagingly, it's a perfectly valid form of storytelling.

The editor is a villain.

I don't remember if they mentioned Meguru's parents being divorced, or was that her mistress?

The sisters "always lived apart", so I believe the separation is assumed.

Who is this?

Same girl as here, I believe.
https://dynasty-scans.com/chapters/how_i_lose_bets_with_my_sadistic_gifted_childhood_friend_and_she_steals_all_of_my_firsts_ch2_2