Nice stuff, leo9. You're far more versed in the history of religions than I am, that's for sure!

today, in the rationalistic heartland of Western atheism, religion is growing exponentially.
I'm confused by this statement. Which "heartland" are you talking about? If you mean the US, it's my understanding that the largest growing segment of religious belief are the "Nones", not atheist, though that segment is slowly growing too, but people not affiliating with any religion. Theists, perhaps, but not religious. If you're talking about Russia, I think a lot of that is rebound from the Soviet era of anti-religion. Can you clarify this?

The third flaw that I can see is the Darwinian theory that religion survives and grows because it promotes cultural stability, hence believers survive and those around them are converted by example.
I've always been under the impression that religions tend to grow most effectively when linked with the threat of death. "You worship as we tell you or we'll kill you" tends to send a particularly pointed message.
most national conversions to Christianity didn't come about by gradual missionary work, but by a ruler's fiat
or at the point of a sword.
If there is any memetic selection going on, Christianity "evolved" not because it promoted social cohesion, but because it made rulers more secure.
Again, I see it differently. Christianity "evolved" because people would no longer allow the Church to kill apostates and infidels, because the needs of the industrial revolution required higher levels of education in the common people, and because it quickly became apparent that the teachings of Christianity were markedly diverging from reality. The story I always liked was about the local church getting struck by lightning while the local brothel was not. Didn't take too long for the churches to accept the "revelation" that God really likes lightning rods!

But we still see the effects of religion on the ruling class. In the US in particular, with very few exceptions, it's almost impossible for an atheist to get elected to any position in government. And any politician who doesn't end a speech with "God Bless America" is almost begging to be impeached.