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...
I for one am not going to rest easy until book seven is at the publishers. The one great genius of Rowling's is always being pitch perfect with the characters. She gets caught off guard occasionally in minute details of the mystery plotting, but the characters always do what, in retrospect, they had to do.
I'm sure that the publishers have a plot outline in a safe somewhere, but no one but Rowling could write it the way it needs to be written. Considering how many times her style has been trashed, I am astounded at how clean the final chapters of this book are. Some of the endings were so confusing I had to read them over just to figure out what happened. This one is a marvel.