...the author intimates that the resulting loss of control was in the overspeed direction.
I think he's straight out saying just that. A Mach encounter due to speed sneaking up to the high side unobserved could lead to a pilot mistaking its characteristics for stall buffet and lowering the nose, further embedding in a Mach tuck pitch-down. Some of those Boeing examples demonstrate how easy it is to mistake symptoms. Disorientation after a Mach Crit encounter inducing a loss-of-control could easily lead to a nose high/stall entry type situation.
Personally not sure about the plausibility of a double flame-out (from a post-disorientation stall/spin scenario) and failure to relight - culminating in an attempted engines-off ditching (as an explanation for the assumed wings level water-entry attitude, high RoD and low speed). The 4 minutes (only) from height could be explained away by the high speed/high RoD required for relight attempts OR that those 4 minutes just represented the time from height to losing all useful electrics (to the ACARS) due to a LOC induced double flame-out.
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