VDF
The normal way (indeed the only way) of unfeathering a Griffon in flight was exactly what you describe ( But you think it would not work )
Move the RPM lever out of the feathering gate, press the feathering button to get the pitch change going, ignition on, fuel on and the engine starts up. You certainly did not start it with the starter motor.
We are getting lots of people on this thread, " sciolists", telling us what they think, and not listening to people who know.