Assalamu Alaykum Wa Rahmatullah
Firstly, marmaduke pickthall , jeffrey lang and hassan turabi are not proper Islamic scholars.
We look at what was said in the actual sources of Islam and the scholars of the past and those who follow Quran and authentic Sunnah in the present.
Firstly, the Niqab was worn by society at the time of the Prophet

. there is ample evidence for it. The Niqab however, is
not obligatory. The Prophet's wives did wear the niqab... but, at the time other women either wore it or did not wear it... Its not stated as obligatory.... and many of the scholars in past and present have written about it.
However, there are proofs for both sides of the argument, and you must go upon what you regard as closest to evidence. The Scholars who i briefly met and know are of opinion face veil is not obligatory..
Please see the following Link for more information about the Hijab:
http://www.muhajabah.com/niqabdalils.htm
The Face Veil Is Not Obligatory by shaykh Muhammad Nassir udDeen al-Albani
At same time we dont critisise any woman who wishes to wear the niqab, as it is their choice and they get reward for it.. Nor should anyone look down at those who do not wear the niqab.. its a matter of choice.
wasalam