Is a mother's refusal to let her daughter stay with her grandmother—for her care—considered an act of disobedience, even though there is someone to care for her, and despite the grandmother being difficult to deal with and invoking curses upon her daughters?

You are not obligated to obey your mother in sending your daughter to serve her, especially if that would lead to harm for the daughter due to the mother's bad character. Apologize to her gently. Serving and caring for parents is obligatory for all children when there is a need.