This is fascinating. Twice Luke tells us "through the Holy Spirit," or, "The Holy Spirit says." What they prophecied came true and so I believe the Holy Spirit did tell them. Should he not have gone? Was God saying don't go? What good did it do to know what would happen and not do anything about it? How does the Holy Spirit direct Paul to go and his friends to beg him not to go?
A literal translation of verse 5 is beautiful: "And placing their knees in the sand, they prayed."
It was old home week. They stayed with Phillip (Acts 6:5) and Mnason one of the early disciples. What sweet fellowship they would have had with saints like that!