Divine Inspiration

I think mainly it has to do with the life those men lived. It's believed they "walked with the Lord", meaning they lived in close relationship with God. Exceptionally so, in fact. I think their works and their messages were also compared to the rest of the Bible to verify their integrity as well, which explains why not all of the 'gospels' in existence were included in the New Testament.

 

Sure, I think it's possible.

A little. Some of the Bible's laws did adjust, even within the few millenia it covers from Genesis to Revelation. Some of it's foundational principles did not, however. That's one of the attributes of Jesus' ministry -- simplifying the old Law into timeless principles that can be applied to a changing world. To use your example, Jesus still condemned adultery but revealed that man was no longer worthy to execute the old punishment for it (i.e. stoning). The Old Testament principle that sin cannot be forgiven without blood remained intact, but what changed was that bulls and goats were replaced by the blood of Jesus. Make sense?