Responses
It was heavily implied, but the specifics were never discussed. Most of what you said is correct from what we've seen, but for the curse undoing, Izutsumi had two curses on her: The soul fusion placed by an unseen sorcerer, and a spell placed by Maizuru to keep her from escaping in the dungeon. Marcille managed to remove the spell but she couldn't remove the soul fusion because it was too strong.
did I just miss something or was izutsumi's backstory never actually explained or is confusing ? So she was cursed by combining her soul with a cat's, and it was implied that it happened when she was much younger and she was sold as a novelty pet or freak show basically. And then she was sold to shuro's family. But when Marcille tried to undo the curse it shows that the head retainer lady of Shuro's family put the curse on her? But wasn't izutsumi already cursed before meeting Shuro's retainers