Were the Gospel accounts based on the testimony of eyewitnesses who were still alive when the Gospels were written?

Were the Gospel accounts corrupted as they were passed on? Was there any way to prevent this happening?

How big are the differences between the hand-written copies of the Gospels' accounts of Jesus? Do these undermine what we can know about…

Did Jesus claim or imply that he was anything more than a prophet? Or did his followers transform him into the Son of God many years later?