[Aratlithiel]

These are all very interesting theories and well-thought-out scenarios. However, I think a better question might be, "what element could you remove from this story without it affecting the outcome?" I believe the answer to that question would be, not a single one.

Think about it...Merry and Pippin HAD to be captured in order for Sauron to believe that Saruman had THE Halfling (and thus, the Ring) and to further believe Saruman had betrayed him. Merry and Pippin HAD to escape and stumble into Fangorn in order for the Ents to become involved in the war.

If Aragorn had accompanied Frodo to Mordor, who would have looked into the palantir and threatened Sauron and gotten him to empty out the lands so that Frodo could get to Orodruin? And who would have travelled the Paths of the Dead gathering much-needed forces for the final battles? These are things Aragorn, and ONLY Aragorn could have done.

Also, if anyone other than Sam had been travelling with Frodo toward the end when he was so heavily influenced by the Ring, I feel almost certain that SOMEONE in the company would have finally attempted to take it from him. Whether it was Ring-induced treachery or simple pity/love for Frodo and the wish to relieve him of the burden, one of them would have taken it away. Sam was certainly the only one in the entire Company capable of getting Frodo to the Cracks of Doom simply because that was what Frodo wanted. He's the only one who never doubted Frodo or his Quest and therefore, the only one who could have helped him the way he did - anyone else present (including Aragorn) would have simply been in Sam's way and would have interfered in the end result.

I would have to go through the book plot-point by plot-point, but I'm fairly confident in saying that even the most minor events HAD to happen in exactly the way they did in order for the Quest to succeed. Any deviation from these events would have resulted in the failure of the Quest, the war and the race of Men. Every single event, no matter how small, has a very distinct domino-effect and if you remove one element, the whole string of events falls and the end result is failure of the Quest.