I'm sure it will all transfer over. Steam is only the delivery method, Impule is the purchase location.
Well I didn't have to touch Impulse, so you clearly don't have to go through it if you don't want to.
Oh god I can see my friends in the gaming circle ridicule me to no end for recomanding Sins Rebellion. First it was SOTS2, now this. I'm done with PC gaming. I'll only play boardgames and whatever I can get on my tablet from now on.
Just play open source games or oldies. They're actually a good amount of them available, maybe they'll last you until a real competitor comes along. If you get heavily into the mod scenes of some of these games it will last even longer.
I imagine you can cancel the pre order through "My Account" in the Stardock store, considering you haven't be charged yet.