What is meant by the "chest opening"? Is it obligatory to cover the entire chest or only the upper part? Is it obligatory to cover it with a headscarf if it is not visible when wearing a jilbab? Is the "muftakaf" head covering obligatory? What about covering the back?

The legal hijab is any covering for the entire body that is neither descriptive nor transparent, regardless of its name or form. It is not a condition to cover the shoulders or back with the same head covering if they are already covered by an abaya or jilbab.

If the chest is small and does not show when wearing a jilbab that achieves proper covering, then there is no harm in that. However, it is still better to let the khimar fall over the entire chest, as stated in the verse: "and to draw their headcovers over their chests." The meaning of this verse includes covering the neck and the entire chest.