Book 1 I think needs few changes. I do think that the eleventh hour loss for Slytherin should be a defining moment for young Miss Malfoy. Her father’s whole schtick could be that “right and wrong are concepts that only the powerless care about”, and the unjust loss could instantly convince her that he is correct. Dumbledore flexed his power and took away something she cared about. Doubly so if her academic achievements have put serious points on the board.
One change I would also like is have her actually attend the duel with Harry. OTL Draco’s fake out here always struck me as weak from an image-obsessed patrician. Of course, neither really hurt the other before the whole thing is broken up by Filch’s arrival
Book 2: the only note I have here is that this change creates a great parallel with Harry and his father. At the end of book 1, Dumbledore tells Harry that Snape never forgave James for saving his life. Rather than be grateful, she would be angry over her debt to this person she doesn’t like.
Book 3: this is where I think you have to make a couple of major changes. OTL Draco’s actions with Buckbeak are a bridge too far. Make Lucius force her to pretend to be injured until he can leverage the incident. At first she’d be happy to play along, but would quickly get bored of pretending to be injured and get petulant with her father. He would lay down the law in a not particularly nice manner to her, and foster resentment. I agree she should be shocked when the execution sentence comes down, and in our next major deviation actually be the one to secretly release the imprisoned hippogriff (mainly to get back at dad but also because she is a spoiled and arrogant child, not a murderer.) Notably, I would not have Harry and co. figure out who did it until at least a book or three later. Daphne Greengrass is basically a blank slate character, so if she needs help you could characterize Daphne as an animal lover and make her Malfoid’s partner in this endeavor. This also enables the complete deletion of the Time Turner plot.
Book 4: the “Support Cedric” campaign can go ahead but I would have someone else run it. I think she would not participate as it’s déclassé but also because she still feels she owes Harry a debt. I agree that she should help Harry at some point with his tasks.
The Yule Ball could go either way. Realistically, at this point I don’t think their relationship is there yet. Plus it’s funny to have both Harry and Ron sitting glumly watching the girls they don’t want to admit they’re interested in dance with others, while the pajeet sisters seethe.
Book 5: this version of Malfoy would absolutely not be down with Umbridge, though for totally different reasons than Harry. She’d be mad about some upstart, simpering bureaucrat being in charge of *her* school. I agree she feeds Harry info - or if you want to get different, Hermione as she knows it’s very unlikely anyone would believe she’d help out a Mudblood - and there’s no way she’d be part of Umbridge’s goons. This marks the cooling of their rivalry and becoming somewhat begrudging friends.
Book 6: yeah, a low point for our heroine. There’s a comic earlier in the thread where the Sectumsempra scene as she’s dying of blood loss is the first time she calls Harry by his first name, which is too good to pass up.
Book 7 becomes almost unrecognizable, but a key plot point ought to be in some way Harry believing in Malfoid and trusting her against all logic, common sense, and the violent objections of Hermione and Ron. Thus their love reaches across the aisle both ways, with Harry believing she still can be good and Malfoy not betraying his trust.