I have no idea what has happened to the helicopter involved and will leave the technical aspects to those that are suitably experienced and qualified to assess them.
With regards to witness statements, I would never dismiss them out of hand, but they can be wildly wrong. A recent report on the Jet2 evacuation at Glasgow airport last year reported that passengers, when interviewed, had reported seeing flames and sparks coming from the engines. The "smoke" was actually condensation from the AC units and the engines were fine. The report suspected that the "flames" were in fact reflections from the anti-collision light on water spray when the reverse thrust was deployed. People under stress, with good intentions, sometimes misread the information that is presented to them.
I have been in the Clutha many times and when a band is playing it is loud. You cannot verbally communicate with the person standing next to you so hearing an approaching helicopter might have been impossible.