I think one key component that some if not all of you are leaving out is the power of the Ring. Let us be clear. In the end, at the Crack of Doom, NO ONE could resist it. No one. Not Gandalf, not Aragorn, and as we see in the book, not Frodo. The will of the Ring rules supreme. Indeed, though Tolkien may think differently (and he has every right to be wrong if he wants to,
), I think that the way everything happened in the book was the ONLY WAY it could have happened, for the Ring to be destroyed.
Things could not have happened differently than they did in the books. Yes, I suppose you can still ask "what if" but in reality there is no "if." IF the Fellowship does not break, then Merry and Pippin will not spark the wrath of the Ents on Isengard, and Aragorn, Legolas, Gimli, and Gandalf will not heal King Theoden and win the Battle of the Hornburg. In which case Isengard will conquer Rohan and attack Gondor. Since Gondor does not have the aid of Gandalf and Aragorn, or of Rohan for that matter, it will fall quickly. And when that time comes, there is no longer any hope.
The Fellowship HAD to break. If it did not, Middle-earth was doomed. Boromir, in trying to take the Ring from Frodo, was saving Middle-earth, though unknowingly. The will of Eru prevails.
OK, I've given the lecture, you all have sat back and listened patiently, now continue with the discussion. It's still fun to ponder "what ifs" like this one. I have not made up my mind, but Fordim's argument for Gandalf currently seems very strong. Pray continue!