You seem to be asking a couple of related questions. Let me attempt to address them individually first, because they interdepend.
How do we Know the Qur'an is Error Free?
As Muslims, we take it for granted that the Qur'an is divinely revealed and preserved, error free, for all time. This is a topic in itself worthy of research. The summary is:
Indeed, it is We who sent down the Qur'an and indeed, We will be its
guardian. (Surah Al-Hijr, verse 9)
This is a promise from Allah Himself, the all-knowing, all-powerful, that the Qur'an will be protected for all time. This verse contains at least five forms of emphasis (the particle lam, repetition, the word inna, etc.) to confirm this. It's a very, very strong statement.
If you're interested in more details, you can read about Zaid ibn Thabit, the famous companion who collected the entire Qur'an, twice, from people who heard verses directly from rasulullah. Not only did he compile this once, but twice, from scratch; that is now the famous Uthmanic mushaf we have today. He (radiallahu anhu) said about this task:
If they had asked me to move a mountain, it would have been easier than this task.
This topic is usually studied in the sciences of Qur'an, called "Uloom Al-Quran."
Who is the Best and Most Impartial in Interpreting the Qur'an?
The Qur'an was revealed 1400+ years ago, into an oral society, in the middle of the desert; for some, on the other side of the world -- that's a long distance, physically and culturally. Civilization as we knew it changed dramatically in that timespan. Not to mention the language barrier, both for non-Arabs and (to a lesser extent) Arabs -- the Qur'an is in fusha, classical Arabic.
Which brings us to your second question: sitting here in an air-conditioned home a contintent away, how can we understand the Qur'an correctly? Who, in fact, would be the best and most objective in understanding the Qur'an?
Given the cultural, geological, and socio-political changes, one would conclude that the people who were there when the Qur'an was revealed* would be the best interpreters. They understand the context, the culture, the language, all of it.
Aisha (radiallahu anhaa) said essentially this when asked about rasulullah. She said:
His personality was the Qur'an.
That is: he was a walking, talking, living, breathing embodiment of the Qur'an. He understood it, lived it, and implemented it -- all of it, without exception, as it was revealed to him and preserved throughout time.
What About Other Humans?
The rest of the answer is easy; if you want to understand who is the best to interpret the Qur'an and sunnah, and Islam in general, the answer is the ones who are most knowledgable and closest to it.
Rasulullah died. He's gone. But he did one very important thing before he died: he taught the Qur'an to his companions, both in letter and in interpretation. They, too, died; but they passed this on to the tabi'een, who passed it on, down through the chain of scholarship throughout time until today.
For us, today, scholars are the ones who are the best fit to understand the Qur'an. Many of them spent decades of their lives studying Arabic, studying the life of rasulullah, the biographies of companions and scholars, and the various sciences (more than a dozen!) of the Qur'an -- in an attempt to give you, to the best of their ability, the real, true interpretion of Islam.
This is why rasulullah said:
Scholars are the inheritors of the prophets. [Tirmidhi, Abu Dawud, Nasai, and others]
What is it they inherit? Knowledge. Real, true knowledge (which necessitates action).
Yes, difference of opinion exists. It existed in the time of rasulullah (he clarified it), and it existed after him; that doesn't take away from what Islam is.