I generally agree with your view of Boromir, but in my personal opinion there were more factors in play. First of all, Boromir lived his whole life in the shadow of war. Sauron was active in Mordor since long before his birth, when he was driven from Dol Guldur. He would have been fighting since he was a young man in a steadily deteriorating situation. Moreover less than a year ago they lost the part of Osgiliath on the far side of the river. As a responsible leader of his people he must have been desperately worried about his city, while the quest dragged on with no end in sight. I think the ring latched on that desperation af an essentially decent man, and that it was a significant factor in his choice to take the ring from Frodo.
Furthermore I believe there is another part to Boromir's betrayal. The simarillion says that even Morgoth had a part to play in Eru's song. Boromir's betrayal gave Frodo an essential push to finally leave the company and head to Mordor. I think that without his actions the quest would have come to a disastrous end; with Frodo either captured by Uruk-Hai or in Gondor in Denethor's hands. If Middle-Earth is a world were fate exisis and if it was Boromir's fate to join the quest, perhaps it was also his fate and necessary that he betray Frodo.
To conclude, I view Boromir as a decent but flawed and desperate man caught up in forces far beyond the control of a single mortal. So I think you make good points, but I have some addenda.
Finally, thank you for sharing your opinion with us and for letting me get my own out in a somewhat coherent manner.