....and locked on to 6000 fpm V/S then 1400 fpm only after the pilot commanded those RoC's apparently independently of the FD's.
HN39,
You are entirely correct that the pilot would have to be commanding a RoC independently of the FDs for the FDs to default to that RoC when the FDs returned (which the FDs indeed do).
The point you may want to consider is that the FD pitch steering will command an attitude that will provide for the specific RoC in effect at the FD return, in fact the current existing attitude, at least initially.
If the airspeed is continuing to decay, the FD will command a successively higher pitch attitude to meet the Roc requirement that was in effect upon FD recovery.
This commanded continual pitch increase is eventually unsustainable....