You have a lot of good points on the Boromir issue, but why must it be an either/or issue? You can be 'of two minds' about an issue, can't you? I see Boromir as both planning to take the Ring and struggling against it. Haven't you ever faced a situation where you know what you should do, and what you want to do, but you have a very hard time doing it? Imagine Boromir who is torn, not entirely between right and wrong, but between two different wrongs. Should he take the Ring, though he knows what Elrond and Gandalf say? Or should he pass it up, even though it is, as he sees, the only way to save Minas Tirith?
My belief is that Boromir is, well, blind. He understands things in terms of force and power. Perhaps he believes that there is no such thing as evil power, that all power could be used for good or evil. Perhaps he thinks that the Ring may well destroy him, but that he can save Minas Tirith first. I am certain that if that was the case he would see himself as virtuous in trying to take it.
That does excuse what he did, or make it right; he was wrong to try to take the Ring. But I think it came as the culmination of a long hard struggle with himself. He waged a personal spiritual battle, and at a moment of weakness, he lost. Once the Ring was taken away, he could become his old self again, and we see what he does - sacrifices himself for Merry and Pippin.
Thanks for your answer. But as to "why must it be an either/or thing", well, because it IS. Either Boromir intended to steal the ring by force from the start, or he succumbed to its evil influence in "The Breaking of the Fellowship". Frankly, I'm a little shocked that nobody seems to see what seems so clear to me in text.