PBY, re:
it really looks that they were still on the autopilot at 3000 feet, as the trim automatically nosed down abruptly to catch the altitude.
Hmmm, not sure I see this scenario. Of course, the elevators pitch the aircraft over to capture altitude not the pitch trim, with the (slower) pitch-trim/horizontal stabilizer following up after, trimming out any elevator control forces, as per usual? Also, it seems to me that
normally-operating autoflight systems don't do anything "violently". They capture "as certified", which would permit 'brisk' but not "violent" manoeuvres, (depends upon the meaning of 'violent', and how 'brisk' is perceived by a tired crew at night). So if it did move, how did the horizontal stab get to the position that is claimed it got to while on autopilot - that's the question.