I find the autorotation to an apparent carpark theory hard to swallow.
Look at Google Earth - that just isn't the sort of area you'd ever autorotate into. A road junction surrounded by buildings and more particularly lighting poles and you know its going to be infested with wires. An auto into that is acceptance of a big crash and unless the situation was utterly hopeless I just don't think you'd try it. They had floats, they were next to/over the river where at least you know wires and poles aren't going to kill you before you've landed.
imo they fell where they did as I can't believe anyone with any control would choose to land there. I think the bar was simply where the accident took them.
Gearbox failure? Two reasons why not. 1) The CAA would have grounded the type if they suspected a catastrophic failure of that kind. 2) You wouldn't try to lift a helo that has had a catastrophic 'box failure using strops around the head.
Tail rotor fail? We think they were in transit to base do we not - so at cruise speed or somewhere close, not in a hover so should have flown right through that suggestion.
The AAIB must have a pretty good idea that it isn't any obvious catastrophic failure or we'd see the type grounded.
What that leaves heaven knows but the witness report of it tumbling sounds credible given the way it landed. One could surmise at lasers or even a birdstrike incapacitation (watch that one, the Clyde attracts gulls at night in swarms) but I rather doubt we'll outguess the AAIB on this one.