@ Bertie
The logic is not entirely flawed, but there are two things firmly against it:
The slots in the fuselage side conform to allowed HS up (max 5 deg) and down (12 deg) movements, and as we have seen on several photos it is framed by strong support beams. That would limit the possible downward movement of the HS in your scenario.
As the HS moves down, the aircraft nose pitches up, and this would be recorded on the FDR prior to loss of sgnal. As the FDR loss of signal was abrupt without anything but normal parameters, the tail separation had to be one of the first events.