1) This part is a bit tricky, because to start with we'd have to understand who Katarina was before the reformation. We do know that she's someone who can attract followers around her, even overlooking class with Sienna's family being barons (which is just a step above being a knight, barely a noble). And also considering her description of her bullying Maria, that she would get so angry without realizing why, it's possible that she was influenced by dark magic in those endeavors (especially since the LN confirms she dies in the Alan route). Considering that in Otome Isekai series the story often includes details not present in the game.
Fiancee's Observation Log has actual magic in the setting when the game made it look like there wasn't magic. Likewise, the heroine of that series has a Light spirit and unknowingly influences the capture targets with it's addictive magic to the point the "love" from the game is compared to being a drug addict.
Duke's Daughter has the heroine being a foreign spy meant to destabilize the kingdom.
Beware the Villainess has the love interests being more idealized in the book than they are in reality. Like Jake going from the guy who shows up to protect Yuri when she's in danger to him being a stalker for instance.
It's a reoccurring element in the genre. We already have Maria isn't the unstoppable genius she appeared as in the game, so who is to say the game's depiction of Katarina is the whole story. This, of course, is simply my theory regarding where things will eventually go in the LN, so take it with a grain of salt and understand how it colors my hypothesis (admittedly, I'm on the fence about it. Like, it's the standard trope of the genre that there was more to the story, but blaming dark magic just feels cheap).
If this is the case, then we can probably get an idea of what a reformed villainess is going to be like. I'd ultimately say she'd be a lot like Bakarina, with her ability to draw people towards her, being kind of an idiot but having confidence in herself despite. I'd say she's likely a bit more insensitive without meaning to at times, with her classism being a bit hard to break out of as without the past life memories she takes her status more seriously. And no doubt more abrasive when she gets upset, as well as possessive.
2) If the above is true, think about it. Her fiancee didn't care for her, using her as a shield to keep away unwanted suitors while eventually planning on throwing her away himself. He fell in love with OG!Maria, and suddenly Katarina keeping a suitor away became a problem. For fun, let's make this Harem route Maria. HRMaria, in addition to going after Jeord, wasn't faithful towards him and ended up with various male suitors clamoring for her attention. She ended up breaking up Mary's engagement to have Alan for example, or she's even getting involved with her family. She starts getting mad, her going after Maria for her commoner background eventually turning into more legitimate issues with her.
Which leaves her open to being dark magic, which makes things worse. She loses control of herself through all this, being a tool in order to keep Maria lonely by Sirius. It ends with her being exiled, Maria is free to have her harem and even picks up the guy who manipulated Katarina into being much worse than a bully. The love interests don't think anything of it and don't look deeper into what's going on in addition to failing to understand just how serious what they did actually was (Keith could be facing being disinherited by Luigi, Alan and Jeord are seen to abuse their powers as princes by the nobility, Nicol's judgement is called into question, stuff like that). And then Katarina ends up in the world of Hamefura.
3) For the other members, I feel like it would be an awkward situation for them. This is a version of their beloved Batarina that has been hurt by other versions them. Sure, she's not the same and isn't nearly as likable, but the fact they would go that far to punish her when she wasn't even in control? It would hurt them. I could very well see Maria being the most horrified by HRMaria, how far she would have gone to no longer be lonely (and even worse if they were to learn about the bad ends of Fortune Lover). Considering how the gang are all friends despite all wanting Bakarina, I feel like the revelations would only serve to make the group closer as they realize how Bakarina has changed their lives and even what they all mean to one another.
As for Bakarina, I see Katarina trying to be a big sister type and trying to the other to understand etiquette, how a noble lady should act, and for the love of god not eating until she pukes.