[CSteefel]

I am still not convinced Denethor knew the whole story of the Ring when he sent Boromir off to Rivendell. It seems to me that there is abundant evidence that he figured things out along the way. And I don't see that Faramir told him everything about his meeting with Frodo, since Denethor makes specific mention of the "half said" and "unsaid" things. But one cannot rule out that his prior knowledge allowed him to interpret Faramir's story correctly. Or, he was able to make use of the Palantir to gain knowledge of the Ring as events unfolded.

This last point leads me to a suggestion, which is that the Ring actually drove Denethor mad, not the solely the struggles with Sauron using the Palantir and the death of his oldest son. Normally, proximity to the Ring is apparently required for it to corrupt somebody, as in the case of Boromir, or Isildur much earlier. But because of his ability to see events elsewhere with the Palantir, I wonder whether Denethor was able to feel the corrupting pull and power of the Ring. Some of his statements in the chapter The Pyre of Denethor are very similar to those made by Boromir just before he tried to seize the Ring from Frodo.

But the key is still the Palantir, which allowed Denethor to be corrupted and finally driven mad by the Ring from a long distance away. The loss of the Ring as it went into Mordor with Frodo may have been the final blow here, more than anything particular that happened with his sons.