Just what was Mrs. Norris' true motivation for insisting the Bertrams adopt 10-year-old Fanny?
Probably to get a future companion for Mrs. Bertram and a desire to look good to her family.
Remember, Mrs. Norris is a vicar's wife: she is supposed to look after the poor and promote family union. It was quite usual for the rich to adopt a child of their poorer relatives. By suggesting it, Mrs. Norris manages to look good while other people spend the actual money.
Check out how Mrs. Norris condescendingly considers the condition of the Price family in Chapter 2... it's not a huge leap of the imagination that she wanted to take away Fanny Jnr (the oldest girl, who should have been the most useful child to her mother, certainly the least encumbrance!) as a way of 'sticking it' to Fanny Snr for being such a slut as to marry below her family's position (Lieutenants of the Royal Marines were not 'gentlemen', for some reason. Don't know why, but neither were officers of the Royal Engineers or the Royal Artillery; and it shows in Price's behavior) (and possibly out of jealousy for her sister having more children than she could afford, when she- Mrs. Norris- could have none.) Routinely humiliating the kid who shared the sister's name with 'kindness' was a way to get back at her while being able to be still convinced, at some level, of one's righteousness.
That is brilliant. It never occurred to me that, by taking Fanny, Mrs. Norris/ the Bertrams were actually making life more difficult for the other sister, not easier.
Or her Inferiority Superiority Complex makes her want to have someone under her in the Bertram/Norris family jointed household ensemble, to treat as an inferior.
Mrs. Norris wanted Fanny to be a part of the Mansfield family, but not as an equal member. That way she has someone to compare with her darlings Maria and Julia (who are older than Fanny and had every possible advantage over her, so yeah, Mrs. Norris, knock yourself out, you evil, evil witch), and thus she can elevate them and make them shine even more. Supported by this quotation:
Narrator: Mrs. Norris had not discernment enough to perceive, either now, or at any other time, to what degree [Sir Thomas] thought well of his niece, or how very far he was from wishing to have his own children’s merits set off by the depreciation of hers.
Just why does Fanny dislike Mary Crawford so much other before she actually does anything bad? For that matter why is she treated as being so irredeemably evil in the first place?
Jealousy, but it's not from the start. Dislike her? She admires her riding and wishes she could ride as well, admits to Edmund she finds her beautiful, is grateful to her for rescuing her from Mrs. Norris, willingly goes to her for fashion advice... she doesn't actively start to dislike her until she starts flirting with Edmund - an inadmirable but very common, understandable, human reaction and really dislikes her after she learns how complicit she's been in her brother's unwelcome pursuit of Fanny herself.
She doesn't. She initially likes her for the reasons stated above. She gives her credit where it's due (defense from Mrs Norris) and sometimes more than is due (necklace affair). Moreover, she speaks well about her (pretty, amusing, likes her very much, likes to listen to her). She only says something negative when pressured by Edmund, and it's an opinion about a behavior (Mary's open disrespect to her uncle) rather than a personal attack. Furthermore, the book outright states that her jealousy doesn't influence her poor opinion and that if she really thought Edmund would be happy with Mary, it would be easier to suck it up and deal with it. She dislikes Mary because some of her "witty" comments are Dude, Not Funny!, because she considers Edmund's favored profession beneath her, because of her involvement in the necklace incident, and because she implies Tom's death would be a good thing.