As someone who was an airfield firefighter prior to becoming a helicopter pilot:
No way in hell would I have tried to physically block the aircraft with an appliance (armoured or not!). [...] I find the criticism of the fire-crews quite harsh and unfair - this was a one-in-a-billion event that they were faced with.
I agree with this - when I first saw the video I also felt like "why isn't anybody doing something" but then I thought about what would I have done had I been there and decided "pretty much what everybody did - wait for it to stop and then lend any assistance I could".
The problem here is that it was already on it's belly (maybe still part of the landing gear?) so the rotor is close to the ground, and it sure seemed to me that it could have tipped over at any point. Anyone under that disk would be asking to be decapitated.
I don't think there is any way you could drown that engine - you could only hit it for a fraction of a second on each rotation - I'd be surprised if you could kill an engine with that amount of water.
It sucks to say nothing could have been done, but realistically, I don't think there is anything that could have been done that wouldn't have risked making the situation much worse than it already was.
As for the cause - one of the videos I looked at seemed like only half the skid was on the dolly, so if that was the case, as soon as the pilot lowered collective it would roll back onto the tailboom which is exactly what it looks like happened. He didn't need to over control the cyclic to cause this - I think once he lowered collective without being sure he was on the dolly he sealed his fate. And I agree with the people that said: "if it was unchocked, why wouldn't you just land on the ramp?". There are the risks we can't control, and the risks we can, and that's one you definitely can control.
And really, landing on a dolly, unchocked or not, why would you ever commit to the landing until you were sure the skids were properly positioned, and even then, you need to take your time lowering collective so that you can abort the landing at any point. It sure looked like a rookie mistake to me. (I used to fly a 206 off a small dolly and it definitely pays to take your time).