Plunders can be done 24/7, as long as there's a convoy going. I've plundered 25mins past the convoy end time.

However, somehow after that convoy event period, even though the convoy had support, it did not activate.

Either he's already been successfully plundered, or he's currently being plundered, or he's more than 15 levels below you.

If not, there is purely no other reason he's not appearing on your list. It's been almost 3 months since plunder/convoy 2.0 appeared and there's no such bug reported.

*Note: Plunder is 24/7, as long a convoy runs, he/she can be plundered even if it's after event timing.

They can run in within 30 mins of accepting that mission. So if they accept it at 8.59, the latest they can run it is at 9.29 (which is 29 mins past the event's close time).

Its not as if they can just run it any time like at 12 midnight or something. It still has to be within the 30 mins windows as like all other Convoy missions.