Originally Posted by Baradium

None of these examples above apply to the scenario and in no way does it apply to a normal green slip because they occur outside of a duty period.

Non flying duty prior to a FDP counts as part of the FDP the same way a deadhead does. If you have only a deadhead you are on duty but since you didn't start duty as a FDP you cannot fly afterwards without 8 hours of rest. IE, the duty period cannot retroactively become a flight duty period, you have to start the duty period as a flight duty period. This is why on a deadhead only duty period you cannot be rerouted into flying. By your logic this would be legal.