What if Glorfindel joined the fellowship to Lothlorien?

[Alfirin]

Could Gandalf have even assumed that? Eliminating Sauron would certainly have taken the commander out of the equasion and probably the generals (i.e. the Ringwraiths) But everyone else in the army of Mordor is not so tied to the Ring or Sauron that His death would ipso facto gurantee thier disbandment. I imagine that, free from Sauron's control , at least some of the Orc legions (especially those already in the west and fighting), would have been recruited by Saruman into his forces with simple offers of continued opportunities to fight loot and pillage. As for your crushing alliance, it might not have been able to muster that group. Fangorn's Ents might simply not be there anymore (Given Saruman's reletless industrialization, the added time free and at large that he would have had while Ganadalf was still on the quest, Fangorn would probably have already been clearcut and the number of living Ents and Huorns near or at zero. Rohan might largely be on Saruman's SIDE if Grima is now King of the Mark (a lot of people might stick to him simply because of that, loyalty to the king overriding any personal feelings) And Gondor might be down to the point where the number of warriors they could muster would be about the same as the amount Arnor could (i.e. about the same number as the rangers) with no guiding force. Denethor would probably be dead in such a circumstance as would Boromir, Faramir AND Aragorn (Aragorn once he got to Minas Tirith, would probably stay and fight as opposed to fleeing even when the situation got hopeless, so the odds of having him to rally behind are remote. Actually since the Rangers would probably have come to Minas Tirith by that point as well, they might all be slaugtered too. The Dead Men could not have come, because Aragorn would probably not have been able to get to them (since in that situation, Dunharrow would be in ENEMY (Rohan) hands. So it would basically boil down to Rivendell and Lothlorien against a Saruman army that might be many times the size it was in LOTR, with massive amounts of extra machinery and explosives (imagine what would have happened had Saruman marched on Minas Tirith too, and simply blasted the walls apart.) I'm not saying you are wrong when you think that Gandalf would say "destroying the ring trumps all" I'm just saying you may be underestimating how much damage Saruman might have been able to accomplish had he been allowed to go unchecked, and overestimating how much of a resistance force would be left to deal with him under that set of circumstances.
Gandalf may have known this. After all when he comes back, he decides that he should stay and help with the defence of the West, as opposed to, say, rounding up the fellowship and heading to Mordor in hope of being able to find Frodo and Sam.
All I am saying is, in some ways, it doesn't make sense for Elrond not to include Glorfindel as Backup, and it's a little odd Gandalf doesn't want him, since having him there would allow Gandalf to not have to make that decision (Glorfy would mean Gandalf would have the option of playing like the hobbit, moving from one to the other as he felt was important, since Glorfy could keep thme safe in Gandalf's absence.

[Zigûr]

In The Return of the Shadow we can see that at several points Professor Tolkien intended Glorfindel to be a member of the Fellowship only to ultimately reject his inclusion: "No Glorfindel" being the abrupt note in one of his planning sections. I feel very certain some explanation was given for this in the notes but I cannot find it. Perhaps I imagined it.

[Inziladun]

Originally Posted by Zigûr

In The Return of the Shadow we can see that at several points Professor Tolkien intended Glorfindel to be a member of the Fellowship only to ultimately reject his inclusion: "No Glorfindel" being the abrupt note in one of his planning sections. I feel very certain some explanation was given for this in the notes but I cannot find it. Perhaps I imagined it.

I found the references you note, as well as looking at all mentions of Glorfindel in the four HOME volumes concerning LOTR. I didn't see any sort of explanation for the change either, but it appears that the next idea for the Fellowship had them all being Hobbits, so maybe that gave the Professor pause to consider other ideas.

[Sarumian]

Originally Posted by Alfirin

Eliminating Sauron would certainly have taken the commander out of the equasion and probably the generals (i.e. the Ringwraiths) But everyone else in the army of Mordor is not so tied to the Ring or Sauron that His death would ipso facto gurantee thier disbandment. I imagine that, free from Sauron's control , at least some of the Orc legions (especially those already in the west and fighting), would have been recruited by Saruman into his forces with simple offers of continued opportunities to fight loot and pillage.

It seems from the book that many of the orcs wanted to desert and live for the sake of there own pleasure. The will of Sauron and the fear of Nazgul was the main binding force behind there armies. Some formations could probably have been hired by Saruman but had he considerable powers over orcs he had not produced, he would probably have defeated Sauron earlier on.

It is extremely difficult to estimate the forces in the case of alternative story. Just if we look at Farngorn - seeing the devastation made Ents to go on war. Though Pipin's intervention help it happened earlier, it would have happened anyway and likely before the most of Hurons were dead. And let me remind that it were Hurons, not Ents who slaughtered Saruman's army at Helm's Deep and later destroyed the fortifications of Izengard.