2 stage collapse
Firstly, I'm a long time lurker - no flight experience, not even on a computer keyboard, so wouldn't presume to posit an opinion on what caused this accident. However I often read witness statements as part of my job, and have to work out how they support and refute each other...
Which brings me to the point about the witness accounts of the band joking about bringing the roof down before the ceiling totally collapsed. Some posters are suggesting this means that the helicopter hit the roof twice, or landed hard and then the roof collapsed under the load. The sketch map of the building suggests that the area of the impact was relatively screened from the area around the stage - it seems to me that these witness accounts are equally compatible with the helicopter impacting once, hard, penetrating the building, followed by a wider collapse as the weight of the wreckage starts to settle. The initial damage might not have been immediately obvious from around the corner, although the noise clearly was. It'd be interesting to know where some of the quoted witnesses were in the bar to see if that held up, but it seems more plausible as a scenario than some of the suggestions.
This idea might be shot to pieces, but equally might save some people from trying to square a circle that eventually turns out to have four right angles in it.
I'll go back to lurking now, and keep hoping to learn new things.