Re: Is hijaab a farz?
My thoughts quickly in brief:
The hadith of Ibn Abbas (RA) refers to "need to go out" meaning "call of nature" in whcih case women are commanded to cover so they cannot be recognised one eye to get bearings this coincides with the other hadith about the Sahabi noticing one of the wives of the prophet (SAW)
A slave Muslim woman and a free Muslim woman are not better or worse for that reason ... in the case of slaves of the wives of the prophet (SAW) they were all Sahabiya or from the tabi'een both of which are good to use as examples ...
The ruling of allowing niqab during hajj by creating a beak/hat to prevent the cover from touching the face is by essence void of the simplicity Islam mandates it is out of character to my understanding and is a fix to allow niqab whilst the requirement for ihram is clearly to not allow cloth to touch the face.
The term "except what is apparent" refers to the appearance for what is apparent if not the aperture, the appearance, the face? The condition of one eye to be open is dangerous as people will not be able to see in 3D depth perception will fail, a woman could trip, unless she is going to an area that she knows well, i.e. call of nature.
Hijab includes atire and behaviour ... atire has certain rules and so does behaviour ... the atire rules are based on aura ... so as long as aura is covered and behaviour is sublime then hijab is observed.