Hello Folks,
I can't really give a informed opinion on Lovecraft or Howard's world views, but as I'm a devout old fashioned Catholic I feel somewhat more comfortable crawling into Mr. Tolkeins. Mind you, this is strictly opinion.
Boromir can be considered redeemed by his realization of the wrongness of trying to gain the ring (sinfullness if you will) and thus spends his life in a valient attempt to save Merry and Pippin, and nearly suceeds. The lack of success is not a factor, rather the ATTEMPT, the INTENT is what matters. In the end Boromir is redeemed both by his selfless actions and sacrifice that comes out of them. Boromir actually would have been redeemed by the first but his sacrifice serves as warning as well. All realize that the Fellowship must divide as the Ring is too powerful a temptation to resist.
Denethor is condemned for multiple failings. His pride, his arrogance, his lack of faith, and his lack of love for his youngest son. He cannot concede that anyone should lead but him, and denies all advice and council. He refuses to follow law and tradition feeling himself above such and wishes to claim the throne as his. This is a great sin as this is a sacred trust. He has looked into the heart of darkness and lost faith in the victory of good over evil, he despairs. Boromir never despaired and rather than see his son's sacrifice as noble one and to built upon (as does Faramir), he sees it as further proof of impending defeat. His lack of love for his youngest, Faramir, is likely his greatest failing. Faramir is at the very least his brother's equal and obviously beloved of Boromir. His father refuses this and destroys himself ultimately. He commits the sin of denying Faramir his due, sending him on impossible missions and then condemns him death on one such for the inevitable failure of another. Wracked with insanity over his realized guilt at Faramir's supposed death he then commits suicide, yet another sin. Sadly, Denethor could have saved himself at any time but refused to see his own failings through lack of faith and hubris.
Gollum seeks redemption and nearly sees it, then sinks back into evil due to his inability to admit he is at fault for his own condition (he did murder his brother in cold blood after all). Hobbits seem very resistant to the One Ring yet Smeagol murdered his brother right off and submitted to the Ring almost as soon as he found it. You have to wonder of the state of his soul from the start.
He does not see redemption in the end, he tries to kill Frodo and take the ring. When he suceeds in the latter, he dies from stumbling into the volcano while dancing his victory jig. Fitting; he dies with the one thing he loves more than himself.
Saruman is brought down by pride and ambition. In the end his own spite at his failure condemns him. he refuses to admit he commited any wrong but failure in his plans. One cannot be redeemed if one does not seek it. Saruman flatly refuses any redemption from the start when given the option.
Opinion, mind you all based on my understanding of the Seven Deadly Sins, the Catholic view of redemption, and how one achieves or misses it. Tolkein was a devout old guard Catholic and as religoius as CS Lewis, just less obvoius about it. So thus I'm working this from that angle, I could be way off.