Physical distribution for Germany, Austria and Switzerland will be done by Daedalic Entertainment, that's already announced.
I don't know if the boxed version of the game will be connectable with your steam account in order to use steamworks. It would make sense to make that possible for easy co-op via steamworks, but not to force players to activate the boxed copy over Steam (DRM?). I don't know if it's possible to give customers the free choice to bound their physical copy to steam or not. Would be great if yes.
I think there will be an update or announcement soon how the co-op will work with non-steam copies (and without steamworks). First there was the statement that you would have to use a virtual LAN connection but then I heard that there might be a better solution for that. We'll see.
