Are you sure about that... Key's sold through Stardock store generate revenue towards Stardock only. Key's sold/registered with Steam require Steam make a portion of that sell. I'm pretty sure that's why Steam doesn't want non-Steam sold keys being registered on their site.
If it means that much though you can probably cancel your pre-order with Stardock and wait for Rebellion on Steam.
No, not sure at all. But with other games I've had and wanted to do this they've been able to simply give me a key that lets me register it on Steam as if I bought at retail, in which case Steam doesn't get any money out of it. That was how I got stuff like AI War moved over.
So it's strictly from a user POV that I talk, I don't know how it works behind the scenes. But I figured it was worth asking. The actual money is going through Stardock's store, all it'd do is let me download/patch in Steam instead of SDC.