Saruman for or against Sauron?

[William Cloud Hicklin]

[Denethor] sent Boromir to Rivendell in the hope he may gain support from the Wise to use the Ring in Minas Tirith, did he not?

I believe you've been led astray by the movies. Denethor knew nothing of the Ring, nor did Boromir or Faramir, when the elder son departed. He was merely enquiring as to the meaning of his strange dream (wherein 'Isildur's Bane' is not clarified, and indeed Boromir thought it might be an Orc-arrow)