Just to add my two penneth worth as an Airbus driver.
The thrust levers in the airbus are fixed, non moving. Hence there is a requirement to closely monitor ALL FMA readings during all phases of flight. A loss of auto thrust or a reduction in thrust, as seems to be the case here, would not be too difficult to spot. Also there is an SOP of 'hands on the top' of the thrust/throttle levers from interception of the final approach.
AFAIK, not being a Boeing driver, the throttle levers move with the commands of the auto thrust and, according to my colleagues on Boeings, the pilots get quite used to where they 'should' be. A prolonged reduction to idle thrust should, surely, have been noticed by either the TC or the safety pilot? One could forgive the trainee as they were, obviously, in their early stages ti warrant a Safety Pilot.
As to the 'horror' scenario of flying a manual thrust/ AP off landing? Not a horror at all, fairly normal in fact. Most airlines allow the Captain to have the latitude to disconnect the AP and/or the Autothrottle for routine line training at their or the FO's wish. Manual approaches are flown regularly at my airline by both seats.
A sad scenario and one that could, possibly, have been avoided. I for one will read the accident report with interest.