[Boromir88]

staying with the Fellowship was in fact morally the wrong course for him to take.

Ahh, then wouldn't joining the Fellowship be morally wrong? Boromir went to Rivendell to get his riddle and questions answered. He did that, so now he was going back to do his civic duty in Minas Tirith. I think Boromir makes an important part of the Fellowship indeed. He provides strength when they need it the most (Caradhras, The Great River), he is one of the best fighters in the company, and he helps Frodo realize he needs to get out of the company before another member becomes a "Boromir." So, there was no doubt of his importance of being in the Fellowship, but you have to ask the question why did he join. If he is wind against the "fellowship sail," always fighting with them, we can tell that he wishes to be the leader of the company, with his words to Celeborn. "If my advice is heeded, we will....but I am not the leader of the Company." Just, makes me wonder if he joined it for the wrong reasons, ok we get the fact he's valuable to the Company, and we know the only reason he went along was to go back to Minas Tirith. With all that happens to Gandalf, Boromir can still leave the company, Aragorn can't. I don't see it as Gandalf's burdens were "forced upon him," Aragorn willingly took on Gandalf's burdens.

If Boromir joined the Fellowship to try to sabotage it, or to steal the Ring,

Interesting you should say that. In one of Tolkien's earlier drafts Boromir actually does join league with Saruman. He lives, goes to Minas Tirith with Aragorn. Aragorn takes claim to the throne, Boromir starts a Civil War, Aragorn kills him before he gets too much support. Now, Tolkien threw out this idea, but you wonder if it would have any relevance/influence on the final story? That is also open to debate.

In any event, my view is that Boromir had by now become so attached to Frodo, or more specifically that which Frodo was carrying, that he would not have left him even had the opportunity arisen. And assuming that to be the case,

That SpM, we just happen to have total different opinions (both ways can be supported). We have both made it clear on what we think on that matter, so no sense in repeating myself. This is of course why I love Tolkien, I wonder if this was his plan to let us make inferences and get our Tolkienists minds going, lol. Sort of like Henry James .