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

Did the early Christian communities apply Jesus's teaching to the problems they faced or alter the facts to fit their agendas?

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

This video describes the discovery of the earliest copy of part of John's Gospel, the 'John Rylands Fragment'.