And the person possessing Rosaline doesn't have some kind of obvious personality disorder... it's not natural for a mother to look at their children that way (and PPD is besides the point as it wasn't like it was in that time frame, she progressively got more and more jealous over time).
She's also possessive to the point that she'll probably try to kill Rosaline and she excuses all her behaviour behind that she *is possessing* the heroine. Emphasis on possessing because she clearly isn't the same type of person the heroine was, she's just a crazy woman possessing the body of the heroine. So basically she's just beautiful.
No she is the same person as the heroin.. She posessed the heroin in the book and live her life until and after the "happy ending" but she was obsessed with Ml's love and atention so after she had his children and he gave love to them she went crazy thinking "HE SHOULD ONLY LOVE ME! I DESERVE ALL HIS LOVE!" So any actin Ml did such as care for kis kids was seen as cheating in her eyes.. To the point she neglected the kids for years and finaly asutled her husband. SHe was locked up in a part of the palaze and wished for a second chance.. She got her second chance aka the timeline of the manga and was expecting everything to be the same but with it not being the same and her being not the "pure self" she was when she first meet Ml.. SHe is pissed and vendictive and maniplative trying to get the "LOve that is her's".
I think the main difrence is that with the heroin its was infaturation, and then developed into obsession.
Where as with FL it was respect for work and personality that grew into love.
Its not just that they got to know eacother, its that they first respected eacothers capabilities and work, then knowing the other persons flaws and making up for it. And getting to know eacother personaly like how the other person thinks, this developed into love in the end as the relationship grew closer. Fl may have started with Motherly love, but when she saw heroin that motherly love blosomed into a crush. And as FL got to interact with the heroin, all her instincts screamed "RED FLAG".
In Ml's case the heroin was the first person to love him unconditionaly, but when they got kids he became caperble of sharing love (heroin didnt like that).
With FL the relationship started with mistrust for her, but as Ml tested her he came to respect her skill and dedication for work. With that they grew closer and he started to nitice Fl's flaws aka overworking and not traking care of herself (getting enough sleep and relaxing). Ml started to make fl rest and even had servants make sure se does so out of concern. This developed into them both knowing the others flaws and personaities and being able to talk to eachother and change eachothers minds (Fl stoping Ml from killing, and Ml stoping Fl from colapins from overwork).
As of now they still havnt realized their feelings for eachother, fl thinks its motehrly love but has specs of a crush now and Ml is caring for Fl but donst know why he does so. All servanst already noticed the sparks. It may take some time as Ml dosnt know what love is and would be slow in the uptake of it. Where as FL never thought it to be posible for the 5 years as she thought Ml and the heroin was destined, but that thought is now crushed due to the heroins rotten personality. Fl is starting to have thoughts such as "i cant leave him in Her hands, He deserves better".
+ Now with the marriage Ml may actualy consider FL as a candidate as he knows she is trustworthy, but will hesistate to bring it up as she respects her feelings and dont wana trample them.