Sage Sankara is the only sage who has final authority on the Advaitic wisdom.+

Sage Sankara’s wisdom has nothing to do with the orthodox sect. Sage  Sankara is the only sage who has final authority on the Advaitic wisdom.

The Advaitic truth is rational truth and scientific truth without dogma.

Sage Sankara’s wisdom has nothing to do with the orthodox belief systems. Some philosophers in the past dissented from this interpretation of Vedanta philosophy, holding that the incarnated Souls were separate from the Divine Essence and only finally merged with it after the cycles of birth.

All these theoretical philosophies are based on the imagination based on the false ‘Self’ (ego or you) within the false experience (waking).

Orthodox people argue that Sage Sankara had a Guru. Sage Sankara himself was Guru.

Yes, for Orthodox people he was JagadGuru but for the seeker of truth he was a Brahma Gnani

Traditionally religious people are so entangled in orthodox religiosity; that it is very difficult for them to free themselves from narrow-minded prejudices dogmas and superstitions. These educated orthodox people are more ignorant than illiterate. They strongly stuck to their inherited orthodox baggage meant for the ignorant populace. Even though their own Sage has said that orthodoxy is meant for the ignorant populace they ignore and they are like blind led by another blind follow the inherited blind belief.

Even Swami Vivekananda was Ramakrishna Paramahansa's disciple. Swami Vivekananda himself said: ~ “You have to grow from the inside out. None can teach you, none can make you spiritual. There is no other teacher, but your own Soul.”

There are two kinds of audiences - the ordinary ones who desire the transitory heaven and other pleasures obtained as a result of ritual sacrifices, and the most advanced seekers who seek to know the ultimate truth or Brahman. 

Guru and Guru -paramparas are meant for the first audience, to help lead its followers along the way. However, there is no need to follow any parampara and follow any Guru for those who wish to realize the truth which is beyond the form, time, and space. 

We should not mix religion with spirituality because religion is based on the ego and spirituality is based on the Soul. Religion is concerned with its paramparas, not truth whereas spirituality is concerned only with the truth, which is beyond form, time, and space. Religion is not spirituality. : ~ Santthosh Kumaar