Seasoned ATR pilot
Fly with Magnar answered to my questions on youtube:
Q: How could they not hear and feel the prop mushing / RPM drop?
A: When the propeller is feathered in flights, the change in sound is more subtle than when feathering the propeller on the ground. And ANR headsets remove almost all sounds from the propellers and engines.
Q: How long would it have taken the prop regulator to unfeather once the pilots would have recognized the mistake?
A: It's faster than on the ground. So around 15 seconds sounds reasonable.