Fact #1 you can still be plundered even after convoy period.

Fact #2 the activity (timed event's) may have greyed out but still you can plunder.

Fact #3 plunder, more specifically the "accept" *on, will grey out if you don't have any remaining attempts left for the day.

Fact #4 plunder is available even before and after convoy period. If you want proof, you can go to your groups "Plunder Commissioner" before or after convoy times and start clicking refresh, but of course you wont see any convoys coz its not yet convoy time.