IMHO the speculation on a spin, flat or otherwise, is misplaced. There is little evidence to support this; all we have is that the aircraft was most likely in a level attitude moving forward as it hit the sea – nothing of what happened in-between, time scale – when, how or why.
I would like to see referenced evidence of any large aircraft in a stable spin; incipient, yes I would accept. I do not know of any such event and this is not for the want of being in conditions conducive to a spin in commercial aircraft.
Aircraft are built and certificated to withstand inadvertent Cb encounters; they do not automatically stall, nor is a spin an automatic consequence of a stall. Large aircraft tend to depart controlled flight relatively gracefully in comparison small aircraft or high performance fighters. The engines do not flame out in Cbs without other contributions, and all aircraft, with their natural stability can transit the relatively short duration of these events in a reasonable stable manner when the controls are used to maintain attitude.
funfly re #3010, the pitot tubes incorporate a leak path – drain hole. See previous posts for likely icing mechanisms. Drain hole blocked = over pressure - higher airspeed. Pitot blocked (before the drain) = ambient pressure – lower airspeed. All other combinations – fluctuating airspeed.