Glad you like what you've seen!
In truth it's actually quite effortful to write and think through decent backstory. I've got notions about the mechanoids and some of the new stuff you'll see in Alpha 5 but they aren't all committed to paper yet.
I wrote the existing backstory on paper during a cottage holiday last summer. It was actually several months before the Kickstarter; I did it because there were no computers around, really. When there is a computer there I always find myself much more occupied with designing the game, writing code, making art, trying to handle all sorts of business distractions - and, of course, chatting with you fine fellows.
I think at some point I should probably carve out another chunk of time to flesh out the backstory some more. I even though of writing short stories and selling them on the Kindle store for a dollar. Might be fun. But again, it's all a matter of finding the time.
As for the rimworld itself, I'm thinking it may not be a specific planet. With future versions of the game likely having a strong element of world generation, which can generate different terrains and different factions and histories (all just theory, but I'd like to do it some day), it wouldn't make sense to say that there is one canonical rimworld where the game takes place.
As for the position of the RimWorld, I imagine 400 years until high-speed interstellar flight, 2000 years of expanding humanity to reach the planet, a few centuries for it to glitter and then collapse for some reason, followed by some more centuries for the ruins to become ancient ruins and for the mechanoids to sleep, before you arrive. The upshot is that it would probably be something like 250-1000 light-years from Earth, depending on travel time, speed, and the directness of the flight.
Of course, because of time dilation and longsleep, people could arrive who were born in ~2010 AD (if longsleep is invented soon enough) or ~3500 AD. That's the fun of putting these two things in a universe together. I really seek a sense on the rimworld of exceptionally diverse people and creatures and entities from different times and places being brought together haphazardly to interact. When spaceflight lasts centuries and people can sleep for millennia it makes for fun cross-cultural/temporal encounters.