1) Depends. Many are uniformed of what the scripture actually says (English Bible only people) and others have a blatant disregard for the unity of all Scripture. Sometimes even disregard for the two verses adjacent to a given verse. I think there is more than one logically valid conclusion on many theologies, but they generally aren't far removed from each other. The filoque for example.
2) No, the Bible isn't the only way. But to get to those other places the Bible is needed.
3) No, it's not clear. The Bible makes it clear that God does not expect us to understand everything about Him. We humans have a hard time separating anything from the flesh, so our understanding of spiritual things tends to be flawed. That is why I go with what we have and then focus on the three greatest things God has given us: faith, hope, and love.