Once you appreciate God’s scheme, the questions that you raised become very trivial.
“it claims to be clear and detailed explanation, but the verses are still not understood properly by the mainstream scholars”
No matter how clear something is, it still needs to be understood by humans, and humans are prone to errors. Let alone any literary works, even something as objective as Einstein’s equation E=mc2 has been interpreted in different ways. Please see:
http://philsci-archive.pitt.edu/2515/
“I am not sure why Allah preserved the text but did not preserve the correct interpretation…”
In the scheme of God, He is testing us in all aspects, even our sincere effort and intellect (38:29). It is not His wish to make things readily evident in His word, just like He did not lay out the schematics of His universe for us in some book. Reward and punishment is not solely based on reaching the right conclusion, but in fact what counts is our attitude towards the truth as we discover it. For those who are conscious of God and are willing to to use their faculties of reason and intellect, most of everything is as clear as their own existence. For what remains, they make continued effort to understand it, and if during the course they find any mistakes in their previous understandings, they correct those mistakes before moving forward. If all scholars agreed upon something, it still may or may not be true. This phenomenon is very evident in the field of science where all scientists agree upon something until a new discovery renders their previous beliefs false.
“Also, many hadiths which explain how Prophet himself interpreted the Qur’an verse show us that these scholars indeed are taking their interpretation from the Prophet himself.”
Please provide examples of objective and material disagreements on verses that prophet SAW reportedly interpreted himself.
“Qur’an relies heavily on the context each verse was revealed in, and that requires knowledge of associated history”
This is not true. In Ghamidi sahab’s Al-Bayan he has translated/interpreted the entire Quran without any historical references.