[Mithadan]

Focusing for a moment on the original post, if Gollum's plot to use Shelob to get the Ring succeeded, an immediate problem would arise. He would be on the borders of Mordor with the only options being to go forward into Sauron's realm, or retreat back towards Morgul Vale (with the Ring). Could he have resisted putting the Ring on (even if he didn't "claim it" as his own)? At this point, probably not. There is a very good chance that he might have been detected by Sauron or a Nazgul. If this happened, all of the war plans go out the window (temporarily). Tolkien actually discusses what would have happened if Gollum had not intervened at the Cracks of Doom and Frodo claimed the Ring and retained It. The same would happen if Gollum had secured the Ring in Cirith Ungol and been detected. All 9 Nazgul would come to him and either kill him (if he did not "claim" the Ring) or feign obeisance until Gollum could be brought before Sauron, at which point the Ring would be taken. If Gollum managed to evade the Nazgul somehow, all of Sauron's servants and armies would have been tasked with finding him. The war would be delayed only until Sauron recovered the Ring.

Let's speculate that Gollum either manages to take the Ring with Shelob's help or seizes it earlier, is not detected and slips away. Sauron's war would proceed as planned. Even if Gondor prevails on the Pelennor as it did in RoTK, this is a temporary setback. The next assault would be successful. Gondor and Rohan would fall. Erebor and Dale would be defeated. Lorien, which fought only a defensive war until the Ring was destroyed, would ultimately be defeated or its people would leave for the West. The same would happen to Thranduil's realm, Imladris and the remnant in Lindon and the Havens.

In short, Gandalf's arguments in the Council of Elrond would prove true. Unless the Ring is destroyed, there is no avenue for the West to prevail.

An interesting area for speculation (and this is all speculation) is whether Gollum would actually attempt to claim and wield the Ring. Clearly, he did not know what the Ring was or how to use it when he possessed it. However, by the time he joins Frodo and Sam, he had been captured by Sauron, and had a better understanding of the Ring, thus his internal dialogue about becoming "strong" and "Lord Gollum" or "Gollum the Great." Gollum was not strong enough to wield the Ring and this would likely only delay the inevitable.