It is a very personal book and so obviously not written by someone else or later that it should be accepted as what Paul himself, along with his associates as you mention, wrote.
I've got to go on a business trip for a few days but would love to discuss this book and what can be gleaned from it concerning Paul, the apostle.
On the subject of Jesus, it's pretty clear that either Paul was mad, or he had indeed experienced the Risen Lord Jesus Christ. There is no alternative.
Edited by randman, : No reason given.