I don't think it would stall, because even at 40 deg aircraft AOA the THS AOA was only about 10 deg. After all, you don't very often see wings stalling at that sort of AOA with plain flaps deflected do you?
Owain,
Your previous proviso that the downwash rule of thumb (0.4 * mainplane alpha) you used still applies in the flow-field behind a fully stalled wing (typ alpha, 35 ~ 45 degrees); nevertheless, expect your conclusion is right though.
Additionally: Significant drag is contributing to NU trim when and if the tail does stall
Overall, there are several here pretty confident now that this aircraft was not
locked into a stall.
That's not to say I would be happy (if asked) with the THS logic at these Mach Nos and altitudes.. this style of 'g' -demand hands-off trimming, is surely never intended for cruise or high altitude flight such as this?