First you have made the claim that Shankaracharya and Gaudapadacharya were in disagreement. This is wrong, for, as I will demonstrate later, this fancied contention of yours is based on your misunderstanding of the meaning of the words 'satyam' and 'mithya'.
All scholars have noted there is a strong difference between Gauadapada and Shankaras presentation of Advaita. They say that Gaudapada presents a pure form of Advaita, while Shankara makes a concession for the householders, by not totally denying their reality as long as they experience it and called this a transactional realty. But Gauapada did not even make this concession, he outright and flatly called it all an illusion
1.7. Those who dwell on creation consider it a divine miracle. Others imagine it is like a dream or an illusion.
1.17. If the phenomenal world were real, it would undoubtedly vanish. All this duality is mere Maya. Non-duality is the supreme reality.
1.18. If the multiplicity were imagined, it would vanish. Such talk is merely for instruction. On knowing, duality ceases.
2.4. Just as dream objects are unreal, so, and for the same reason, objects perceived in the waking state are also unreal. The only difference is the restriction (of dream objects) to an interior location.
2.6. What does not exist in the beginning and does not exist at the end certainly does not in the middle! But like illusions, they seem real. (Even as an object appears it is decaying in time, such that it is not what it appears it is receding into the past, while that which is conscious of it is never bound in Time.)
3.27. That the existent should come into being is only possible in an illusory manner. Anyone who claims it really happens is saying that what has already come to be comes to be.
3.28. The non-existent does not come to exist either in illusion or in reality. The son-of-a-barren-woman is born neither in illusion nor in reality.
3.26. Consciousness has no contact with objects, and no contact with appearances of objects. Objects are non-existent and appearances of objects non-different from consciousness.
3.27. At none of the three times (past, present, future) does consciousness make contact with objects. Since there are no objects, how can there be deluded perception of such?
3.28. Neither consciousness nor its objects ever come into existence. Those who perceive such a coming-to-be are like those who can see footprints in the sky!
This is known as
ajati vada, the doctrine of no creation. Gaudapada was a staunch proponent of this and made absolutely no concession of there being even a temporal creation. He was the teacher of the teacher of Govinda, Shankara's teacher. So if Shankara is just a disciple continuing the tradition of his teachers, why would he teach the opposite of ajati vada? Shankara didn't at all, as we have already see him explicitly state in his Vivekachudamani earlier many times. Here are just a few instances:
1.20. A firm conviction of the mind to the effect that Brahman is real and the universe unreal, is designated as discrimination (Viveka) between the Real and the unreal.
1.63. Without causing the objective universe to vanish and without knowing the truth of the Self, how is one to achieve Liberation by the mere utterance of the word Brahman ? -- It would result merely in an effort of speech.
3.232. If the universe, as it is, be real, there would be no cessation of the dualistic element, the scriptures would be falsified, and the Lord Himself would be guilty of an untruth. None of these three is considered either desirable or wholesome by the noble-minded.
3.234. If the universe be true, let it then be perceived in the state of deep sleep also. As it is not at all perceived, it must be unreal and false, like dreams.
It really does not get anymore explicit than this. I have now shown you categorically both Gaudapada and Shankara flatly denying the existence of the universe and say that it vanishes completely like a dream on realization. How you can interpret this not to mean this is flagrant dishonesty. I really hope our objective readers reading can see how dishonest you are being by pretending Advaita does not teach the universe is an illusion. For I have now cited 3 core Advaita works: Panchadasi, by Vidyaranya, Vivekachudamani by Shankara and now Mandukakarika by Gaudapada and everyone of them have explicitly stated the universe is an illusion that disappears like a dream on realization.
Second, and the main point I wish to make, is your claim that Shankaracharya's works do not define Advaita. Shankaracharya is, to my knowledge, the only Advaita Acharya who has commented on all of the major Upanishads, the Bhagavad Gita and the Brahma Sutras. For this reason I have stated that anything which contradicts Shankaracharya's words contradicts Advaita. If our views regarding Advaita militate against Shankaracharya's own words, then it is not the Acharya's words which we must reject- why? Because Shankaracharya is a sure authority, a reliable authority.
I am going to make two points here
1)Appeal to authority fallacy. Shankara's authority is not infallible, he is not a apta(Risi) his works are not divine revelation(Sruti) He is an ordinary human like you and me, and he has imperfections like you and me too. Moreover, Shankara did not start even his own tradition of Advaita, the tradition was started by Gaudapda his teachers teacher. It is a well accepted fact within the Advaita tradition that Advaita is not a new philosophy which Gauadapada invented under the influence of Buddhism, it goes back to the Upanishadic seers, one of them Yajnavalkya, is regarded to be the earliest and most representative Advaitist by the Advaita tradition.
Nor were the first prakarana granthas first written by Shankara, pre Shankara Advaita parakarana granthas exist like the
Astavakra Samhita. In the introduction to the Astavakra Samhita translation by Swami Niyyaswarupananda, Dr Satkari Mukherjhee, former professor of Sanskrit at the University of Calcutta and expert on Indian philosophy, writes:
The astavakra Samhita is an accredited classic of Advaita Vedanta. It leaves no room for loophole for the misconception or misinterpretation of its fundamental standpoint or attitude. It would have been a matter of great satisifcation had we been in possession of similar works in other schools of philosophy and religion, the paucity of which is poignantly felt by scholars regarding the Buddhist school. The present work makes us understand what Advaita Vedanta stands for. Its assertions are categorical, positive, direct and unambigious. It is an ancient classic, which proves that Gauadapada or Sankarcharya did not propound a newfangled philosophy under the influence of Buddhist idealist thought, as facile scholarship seeks to estabish now days in the basis of superficial and flimsy parallelsisms. This system of thought, on the contrary, had its moorings in the Upanishads and received definite articulation in older works, including the present one...It gives a candid, unhestitating, unblushing and unapologetic presentation of the cardinal principles of Vedanta, without consideration for and concession to our realistic predispositions. No wonder, that it is a vade-mecum for monks of the Sankara orders. Of course it has not been widely popular with householders because it refuses to cater to their instincts or to make allowances for the exigences of social or political circumstances
Now let us see what the Astavakra Samhita says:
2.1. Oh, I am spotless, tranquil, pure consciousness and beyond natue. All this time I have merely duped by illusion
2.2. As I alone reveal this body, even so do I reveal this universe. Therefore mine is this universe, or verily nothing is mine
2.3. Oh, having renounced the universe, together with the body, I now perceive the Supreme Self through the secret of wisdom
2.16. Oh, the root of misery is duality. There is no other remedy for it except the realization that all objects of experience are unreal and I am pure, one, consciousness and bliss.
2.17. I am Pure consciousness. Through ignorance I have imposed limitation upon myself. Constantly reflecting in this way, I am abiding in the absolute
2.18. I have neither bondage nor freedom. Having lost its support, the illusion has ceazed. Oh the universe, though existing in me, does not in reality so exist.
2.19. I have known for certain that the body and the universe are nothing and that the Self is pure consciousness alone. So on what is it now possible to base imagination
All pre-Shankara Advaita texts make no concession to houeholders, they are only for the renunciants(sanyasa) those who completely reject the world, treating it is an unreality and source of delusion and misery. This tendency can be seen up to Gaudapada and thus we find Gaudapara is pure Advatist. The Bhagvatpada was started by Shankara, who unlike his predecessors makes a concession to householders and legitimates their worship of Ishvara as means to an end of higher Brahman-realization. He advances the theory of vyavahrika or transactional reality in order to accomodate the reality of others as well. Why did Shankara do this? Shankara was a missionary and his aim was to bring Advaita Vedanta to the masses. Prior to Shankara, Advaita Vedanta was only for the renunicant.