Salaam Dear Brother
You are so right! The Sunnah certainly account for part of the problems, as they can be easily manipulated, and added. It is not only ignorance, but also Greed by the self imposed leaders of those communities, that follows assumptions and false teachings.
I have no doubt that The Book has all I will ever need. If I needed more My ONLY LORD would had dictate in it.
I believe that the Prophets were just delivering The Messages, they were not teachers, and they were human like us.
No one has enough knowledge to add or alter anything of The Book. If anyone try to slot a phrase or change a word in it, this will become evident and easily detectable, and changes will alter the flows of the messages and it will be contradicted in other sections of This Living Book, just like it happen with the Torah and The Bible.
The more I read The Book the more I realise How Mighty that My Lord Is (and I realise that I am much less than a speckle of dust).