Documented Chains. Not single orations, but hundreds of memorizers before a strong chain was declared. There are several dialects in which chains back to Prophet Mouhammed (PBUH) have been confirmed. 7 (don't quote me on the number please) remain considered to have strong chains. Of these 7 the number of letters are exactly the same. The words are counted different (again to the other discussion, dialects) there are several others that are considered to have weak or non connected chains (means there weren't enough people who were documented to know it at some point that the chains became too few for a generation or more). None of those dialects and alternate meanings in those dialects from that time differ from any others. I have a friend who is looking to get his recitation certified in the next couple years (as he finds the certified person's time available) He has memorized 2 dialects and can recite them clearly and properly. 1 he has certified, but is looking to get it from a shorter chain certifier.
as for the ones that were burned. It is well known that in the early days of Islam that people were eager to spread the knowledge they had gained and memorized of the Quran. Since there wasn't documentation to many of those non- companion/ Imam people spreading, the words were misunderstood and recorded incorrectly. When the Quran books were ordered to be collected, they were compared to multiple reciters and if even 1 letter was off, the book was deemed a bad copy and incinerated. Most of these were incomplete. You may choose to not trust people based on modern issues with trust, but if you have 1000s of books and 1000s of reciters that have matched for at least 1300 years, why would you assume it were changed? It is true the first completed book was lost (burned), and the first copies were lost(burned and degraded beyond recognition), but the copies of those are still in existence, and those were overseen by the people who were companions of the Prophet (PBUH), and certified by oral dictation by many people.
Salaam Allahikum, and May Allah open your heart to the truth so you may realize the light.