"This requires assessment of the flight instruments, some of which were not available / accurate in AF447"
As I understand it, at least the standby AI (if not the main AI) was functional.
I would have thought that the sight of a lot of blue might have been a hint that they had a rather excessive NU attitude and that consequently their rapid descent might indicate a stall.
Admittedly the main AI may have been almost obscured by alarm messages but from what I have read the standby AI was working.
But what do I know......?