Great questions but they're all too fundamentally flawed to be of any real use for anything other than arguments along the lines of "my religion is better than yours."
2 Tim 3:16 doesn't define what constitutes "scripture" so that's useless. Paul doesn't provide a list for an accepted canon that he is referring to, but since the NT hasn't been written yet, it can only refer to Jewish scripture.
Q2 - we don't actually really know how the apostles viewed scripture as we don't have their written comments on the subject, just hearsay by anonymous authors.
Q3 - this question is based on a problematic premise - that the Bible teaches anything "clearly." It is too full of contradicting ideas and statements. Most Christian doctrines are based on short pithy cherry-picked quotes or a mash of conflicting arguments.
The Bible is an extremely poor piece of communication if it is meant to convey divine doctrines, and no-one could ever claim that their interpretation is the closest match to what the author intended, so the whole idea of Bibles clear teachings vs religious leaders teachings is a misnomer.
Q12 - short of a divinely signed document this is impossible to verify, but so is the mandate of the Bible writers themselves - there is no way to verify that they were authorized by God to write what they wrote, so again the mandate of interpreters is meaningless.
So to sum up, no-one really has any authority to teach anything about God, assuming there even is one.