Is it sufficient for a Muslim woman to wear a khimar and a dir’ while it is permissible for her to uncover her face, or must she wear a mila’ah that covers her completely from head to toe, otherwise she is sinful?

Any attire a woman wears that fulfills the conditions of the Sharia-compliant hijab is permissible for her to wear. It is not a condition that she wear a 'mala'ah' (a full-body covering) or an 'abaya' (cloak) from head to toe.