[Rumil]

I must admit I'm still wondering about the relationship between Saruman and Denethor.

Regardless of putative palanti-ing, Saruman was noted as being more concerned with Gondor than Gandalf, who was more involved with Eriador and the North of Middle Earth. Saruman visited Minas Tirith fairly often, as Denethor iirc commented when Gandalf came to search for the scroll of Isildur.

It would be natural, if Denethor trusted Saruman, to direct Boromir first to Rohan, then to Isengard, before trying to find Rivendell. If this had been done Boromir's journey would have been easier and he might have been able to get some information. Surely Saruman would be one of the best people to ask, as although Denethor might not have known he was on the White Council, he was obviously learned and long-lived, and on the way.

For some reason Denethor decided against asking Saruman, I reckon he must have been suspicious already.

I wonder if Saruman ever met Thorongil?

I guess the location of Rivendell was known to Denethor in very vague terms - ie somewhere near the end of the Great East Road before it crosses the Misty Mountains, but as has been said, its exact location was hidden. Even so Boromir lost his horse at the crossings of Tharbad, rather out of the way as the crow flies. Was he therefore heading up the Greenway to Bree and turning right, or did he cut across country up the Hoarwell? If he went through Bree, I wonder how this fits in with the timing of Frodo's journey?