Enlighten me as to why the feathering of the tail would be more dangerous at mach 1 that at 1.4 ?
Feathering the tail is dangerous at any high speed in dense atmosphere.
Unlocking the feather, however, may well be more dangerous at around mach 1 than 1.4. Reason being that the airflow may be less stable at this speed as you're in the trans-sonic regime.