I disagree with some of the analysis of Denethor, and agree with a lot of it. Denethor as you quoted was the greatest man in his line for generations, and had the Numenorian vice of excessive pride in himself and his wonderful city. Like many fathers he put his hopes on his firstborn son, but the name he gave Faramir shows both his obsession with Boromir and his inadequate qualities as a father (something not uncommon with rulers, if you remember The Lion In Winter). It would really have grated on him that his father gave preference for an apparent commoner from the northern wilderness.
Then in his pride, he thought he could beat Sauron himself, with the palantir. Over the years he would have fallen under Sauron's power almost as thoroughly as if he had born the Ring. Then he discovered that the son that he doted on was killed defending the (to him) worthless halflings. This drove him over the edge into insanity. His anger turned on Faramir, who in turn appeared to have been mortally wounded. His guilt and grief over this drove him totally insane.
But it was his own sins and mistakes that had brought all of this about, the responsibility was his, although Sauron and Saruman had added to the disaster. In his insanity he totally forgot the duty he owed to Middle Earth, and to the people of Gondor, and acted to indulge his despair, rather than to fight evil, to the death if necessary.
The way I understand it, the unforgiveable sin of Judas was despair, he felt that his crimes were too great to be forgiven. With all of his faults Denethor was redeemable, until he indulged in the unforgiveable sin of despair.
The sin of Judas was greed.
Matthew 27 said that Judas when he saw that Jesus was condemned (not in Judas' plan...) he repented and went to be forgiven by the Sanhedrin. They shrugged their shoulders at him telling him to see to his own sin. In anger, he threw thier money back at them... then he hanged himself.