Sri, Sankara says in Brahma Sutras: that Brahman is the cause of the world, whereas in Mandukya he denies it.

One finds lots of differences between traditional Advaitic preaching and practice. There is need to bifurcate religion, concept of god and scriptures from Advaitic philosophy to assimilate the essence of Advaita.   

Sri, Sankara says in Brahma Sutras: that Brahman is the cause of the world, whereas in Mandukya he denies it. This is because he says that at the lower stage of understanding, the former teaching must be given, for people will get frightened as they cannot understand how the world can be without a cause, but to those in a higher stage, the truth of non-causality can be revealed.

Sri, Sankara himself has warned us not to use ambiguous words, and to practice semantic analysis in his book "Definition of one's own Self. [" Page 199, v.24 of "Sankara's Selected Works]

Sri, Sankara founded his Advaita Vedanta either on reason independent of sruti or on sruti confirmed by reason."   Sri, Sankara's commentary on the Mandukya Upanishad, II, 1:  This [the unreality of duality] is borne out by the Srutis ... But it is possible also to show the unreality of the object world even from pure reasoning, and this second chapter is undertaken for that purpose.

 Sri, Sankara himself had often said that his philosophy was based on Sruti, or revealed scripture.  This may be because Sri, Sankara addressed the ordinary man, who finds security in the idea of causality and thus in the idea of God—and Revelation is indispensable to prove the latter.  He believed that those of superior intelligence, have no need of this idea of divine causality, and can therefore dispense with Sruti and arrive at the truth of Non-Dualism by pure reason. 

Sri, Sankara, in debates with Buddhists and others who did not recognize the authority of the Vedas, had been obliged to prove the truth of Advaita by means of reason alone.  Mandukya Upanishad, a scripture which appealed to reason to the exclusion of Revelation. 

  Non-duality does not need the support of any Scripture or Revelation like the Veda. For it is based, not upon the varying theological fancies, which are as numerous as the sands of the sea, but upon reason, the common heritage of all mankind, irrespective of colour or creed or clime.

Thus, self-knowledge is meant only for those who have intense urge, and courage to accept the truth with humility and reject the untruth. Since people start comparing with their scriptural knowledge, it becomes impossible to assimilate and realize the non-dual truth.   Therefore, there is no need to convince anyone other than our own-selves to get the firm conviction. 

 I am not trying to prove my views, but it is for every seeker who is in the path of truth to prove himself to know “What is truth, “and “What is untruth?” to assimilate the self -knowledge. 

When one is unable to quench his spiritual thirst from his guru or teaching he has to look elsewhere to clear the cobwebs of his understanding and to move forward in his spiritual quest.