Is it permissible for the "isharb" that covers the hair, neck, and chest to be a legitimate Islamic head covering (hijab) when worn with an abaya, and why is it not permissible if it does not cover the shoulders, even though the Qur'an did not mention covering them?

It is permissible to suffice with a piece of cloth that covers the head, neck, and chest along with an abaya, if it covers the rest of the body. This is considered a legitimate hijab. It is not a condition for the aforementioned piece of cloth to cover the shoulders if they are covered by the abaya. The legitimate Islamic dress does not have a specific form, rather it has conditions and specifications that must be met. Allah Almighty commanded the covering of 'awrahs (private parts) in general, and commanded women to draw their head-coverings over their bosoms to cover their chests, necks, and ears with the head-covering, as stated in His saying, "And let them draw their head-coverings over their bosoms" (An-Nur: 31).