Sorry to be pedantic, but I understand that the aircraft was check flown after the incident by 2 'real' (ie graduate) test pilots, who, like the engineers, were unable to find a fault with the aircraft.
It's quite easy to get 'in the loop' with the 412 in AP-out spot turns, but I had not heard of a vertical PIO. I wonder whether it started out as a bit of roll oscillation, which excited some yaw, and then coupled with a bit of heave, which got out of hand? Only 3-axis Helipilot, methinks, so can't blame the collective channel. And I gather that re-engaging the AP stopped it all.
Lesson: Don't taxy in AP out (especially with a tailwind..and NB critical azimuth..?) when you're about to RRF. There's a time and a place for practising what should be a 'degraded mode'.
Ah: the Force Trim! Another old chestnut. Personally, in high-gain situations, either 212/412: FT OFF. That means, basically, below about 250 ft. Above that, FT ON. Then ATT mode as soon as possible. Philosophy: use the FT and Helipilot system as it was designed to be used on the type in question, not because that's how another type's is used when you pop out of the flying training empire.