LotR: What If Saruman Doesn't Join Sauron? *spoilers*

You’re missing the whole point of Saruman, and maybe even the entire trilogy. Yes, that is exactly what Saruman was saying, and may even have half-convinced himself, but it was really all motivated by a desire for power.
And that is, I think, one of the central messages of LOTR. Sure, an acknowledged super-evil like Sauron is bad, but it’s easy to recognize that. The real problem, the real thing we need to always fight against and be on guard against is those who see war and conflict as avenues for gaining personal power, either temporarily like Boromir (and even Frodo) or more permanently, like Saruman. Seeing through their rationalizations is far harder, but far more important, than knowing Sauron is bad.

Manwe only picks Gandalf.
Gandalf does not want to go, he fears he is not strong enough and is afraid of Sauron.

I guess it depends on perspective and if you encompass all the poems and writings etc.

Impatience begins the fall into shadow, it is out of tune with the song.
Even evil must be dealt with in proper accord and proper time or it leads to downfall.

Remember, the Valar did once not handle things in Eru’s way, and the outcome of that is what brings us here to this point. (In the story not the forum or course)

What, precisely, do you mean by “did once not handle things in Eru’s way”?

Do you mean you think they should have called on Eru to solve things at the end of the First Age? That this would somehow have changed what happened?

If so, I don’t agree. The reason Eru steps in and changes the world at the Akallabêth is because the younger children have violated the terms of their existence, which is set fort by Eru himself. They are not allowed to live forever, and, thus, are not supposed to go to the undying lands, at least not in large numbers (it’s amazing how many mortal men end up going to the Undying Lands in the end!). Since it is Eru’s law being broken, the Valar step aside and ask Eru to solve the problem.

But at the end of the First Age, Eru’s edicts are not being violated. Indeed, since it was the Ainur themselves who played the music that foretold the First Age and its happenings, it’s very appropriate that they work to clean up the mess they made by inviting the Firstborn to the Undying Lands. Not sure what you expect Eru would have done differently; Melkor/Morgoth is banished from the circles of the world, and while the entities who served him survive, they did so after Eru acted at the Akallabêth, too.

It’s not that “mortals are forbidden in Valinor”. It’s that the Numenoreans specifically were told that Valinor was not for them. Going to Valinor doesn’t give you immortality, or break the world. Yes, searching for immortality doomed Numenor, but invading Valinor was delusional. That was why Numenor was destroyed, because they tried to take by force something they could not have.

And of course all the mortal stragglers who ended up going to Valinor at the end of the third age lived out their lives and died there in peace, they didn’t become immortal.

The invasion of Minas Tirith wasn’t done by Sauron’s whole force, or even half his forces. Sure, it was a setback, but armies of orcs and men are easily replaced, Sauron’s biggest lost was the destruction of his powerful slave, the Lord of the Nazgul.

Post 4 here may be of interest: We Need Your Votes!! in the Anthology Thread of the February 2012 SDMB Poetry Sweatshop! - Cafe Society - Straight Dope Message Board