Surely all the extensive discussion on autopilot type and modes selected is secondary here?
This sort of approach is presumably fairly routine in the NS and shouldn't have caused any problems. Autopilot coupled on LOC and VS, pilot sets collective to achieve target AS, adjusts if necessary. Both pilots focused on critical part of flight, standard scan focuses on airspeed, altitude and radalt, as MDH is approached monitoring reaches max, a go around ready to be implemented if necessary. Not complicated, surely? Workload is reduced with the LOC and VS coupling.
Events going wrong didn't happen quickly here either - the airspeed was gradually decaying over 60 seconds or so - plenty of time to see things going awry with decaying AS and heights too low. And it wasn't as if they hit high terrain from loss of situational awareness (eg Mull of Kintyre). The crew knew it was flat sea between them and the airfield and that their critical input was to monitor height and go around if required.
When more information is revealed it will also be interesting to see what happened in the final 200ft or so, when presumably the crew became visual. Did they pull max Tq and try to recover? Or was local vis too bad to properly get visual before hitting the sea was unavoidable. The crew will certainly know.
And yet despite not just one but two experienced crew, this basic monitoring appears not to have happened. Again it is hard to think they simply were not paying attention for such a long time. Could they have been incapacitated somehow? Unlikely, especially both, again. Did each pilot wrongly assume the other knew what he was doing?
And why did the airspeed drop off so much in the first place? Could that have been from something as simple as collective friction not having been set at the top of the descent and it edged down un-noticed? (Never flown SP so no idea if relevant) Of course monitoring and reacting is still the critical bit.
Was there a major distraction? Warning lights? Fire? Passenger interference?
Quite a conundrum.