What is the ruling on wearing backpacks over the khimar, and must they be worn underneath it, keeping in mind not to delineate the shoulders?

This type of bag is permissible for both men and women, but it is not permissible for a woman to use it in a way that describes the size of her body or highlights her allure. Rather, she must observe the modesty required by Sharia, and her clothes must be covering and not describe her body.