In the Mac version, double clicking on the icon in the corner of the Index Card opens the document in the Editor. Does that not work in the Windows version?
Hmm, there should be a Documents > Open > in Editor command with a keyboard shortcut that should be customisable, but you are right, this is currently missing. I’ll pass this onto the Windows team. As Martin says, though, in the meantime double-clicking the icon in the corner of a card will do what you want (although obviously that’s not a replacement for a keyboard shortcut if you want to avoid taking your hands off the keyboard).