“…why the PF and PNF did not action the UAS…”
Perhaps they did, except that it was the memory drill required for after takeoff – nose up (15 deg?), as opposed to the more benign level flight case on the same page.
This action might have been influenced by an inappropriate emphasis on this part of drill during the mandated UAS refresher training; also this might identify a difference between the accident and other successfully recovered incidents, apart from many other human behaviours.
“…the apparently deferential attitude of the older, more-experienced PNF…”
Perhaps he too was struggling to understand the situation, and with rapidly evolving circumstances was mentally limited, including time dilation, which delayed alerting the Captain.
The lessons to be learnt from this accident are in the successful recoveries of previous events. Everyone is trying to find a cause and thus something to irradiate, yet if we can understand what previous crews did with success and incorporate/strengthen that in operations then perhaps we might avoid similar situations.
W.r.t. the many French agencies, I have worked with most of them during certification and safety-incident investigation. My experiences were of very dedicated, knowledgeable, impartial, and independent organisations and individuals, whose primary aim was safety.
In recent years investigative authorities tend to invite the aircraft and vendor manufacturers to take a much more active role in investigations as it is apparent that no one group can have a sufficiently deep understanding of systems design and operation.