I think you are right, but if so, it was planned.
I really am convinced it was planned out just as it happened, with some surprises--I don't think DD had any idea how Draco got the Death Eaters into Hogwarts, he seemed pretty surprised about that. I'd love to spend a few hours in the Room of Requirement. Just walk in and find a copy of book 7 sitting next to a comfy chair...