Loss of altitude without a commitment to STALL means exactly what the PF kept reporting, "I feel we have some crazy speed." If at a/p drop, his NU was to arrest descent (as he sussed it), and the 'descent' never 'arrested', this easily explains his commitment to overspeed in descent. It explains his deployment of spoilers, and his disregard for the STALL WARN; without comment or interference from his mates, it appears they all bought it.
But fails to explain why TO/GA thrust was selected, no? Where does that fit?
And the Captain restrained second use of the spoilers right?
What of the altitude call outs/VSI?
If you selectively fit the jigsaw pieces together then may be you think you have a picture, but the box is not empty is it?