Surely the idea that a chopper pilot would choose to land on the roof of a building when he could land on the ground, even a busy junction, is somewhat strange? Cars and pedestrians would be likely to have at least some chance of getting out of the way, whereas those in a building would not, not to mention the added danger to the crew if the roof gave way.
Also surely the C of G of this craft would mean it stays upright when the blades are seized?
I believe (from one photo I saw) that there is an area of open ground, albeit not very large, immediately to the rear of the pub. It's possible that this dark area was chosen but the dark coloured roof was indistinguishable from the open ground until too late to avoid.
We really can have no idea how a "stopped rotor" EC135 would fall to earth. Again, no-one has yet volunteered to be the test pilot for this.