I know a great deal of work was done to establish the state of the aircraft systems at impact,
It was indeed. Almost in it's entirety by an independent group of World leading experts. The Air Accident Investigation Branch.
If you will allow me to summarise their findings, they stated that the pre-impact serviceability of ZD576 could not be positively determined.
You invite us to correct you if you are wrong in your assertion that:
everything was working
All I can say is. After the 'great deal of work' you correctly mentioned, the AAIB were unable to agree with you.
It really
is that simple!