It looks, Denethor at some point came to the idea that the treacherous Grey Wizard was the sender of the dream... And I please myself thinking he was correct.
What Denethor could never appreciate is that someone else was able to take better care of Gondor. I would like to bar powers far away from involvement into every tiny thing happening in ME. In my version the dream was sent by Gandalf but the verses were most probably written by good old Bilbo. Why Gandalf needed a son of Denethor coming to Rivendell? Simply because it was Elrond's land, where, out of Denethor's reach, the stranger would have paid more respect to His Majesty unmasked. He initially wanted Faramir to be the witness for the coming king. It was in vain, however, as Faramir's father did not believe dreams as other ultimate rubbish. So Gandalf changed his mind in favour of Boromir. He could have thought it was even better to convince the stubborn elder brother to embrace the king returning as he was able to put pressure on his own father. With the Ring in proximity it all came up even more complex but worked well in the end (not for Boromir, unfortunately, but I believe, Eru redeemed him). And if Gandalf appears a bit machiavellian this way, it only gives more insight into his character and circumstances. He also was slightly ruthless when he persuaded Thorin Oakenshield and Bilbo to go for their dragon hunt, wasn't he?
