It's currently only available for backers. I can see from your group title that you're not a backer, unless you didn't redeem your forum badge, so you have to wait along with those of us who refuse to put up with Steam.
I don't actually know if that's true. The UPDATE is for backers, but I don't know if RTG can differentiate backer Steam keys from non-backer Steam keys for that purpose? ....I have no idea...
You're right, but you don't have to redownload. Just switch the version and restart steam.
Good point. Should have made that clearer.