When I see the animation it looks like the defender bombards, and then the attacker moves forward. Hence, it is the attacker's tile square, presumably with that tiles terrain bonuses. Of course, the findings about defensive bombard would affect the current combat calculators. But all this seems so insignificant to me. Defensive bombard does not really tip the battles too much in or against your favor, so the situation must be really bad for one to need to worry about it. And if one is relying on this for AW, then good luck winning at all. (Not to say that one should do without defensive bombard though. A simple approximation of its effectiveness would get the job done much faster.)
But, of course, if you have the time, by all means test this for us.