The convoy.mis is ever present throughout all the mission projects so ships will still be spawned from that file whatever mission you are in
Not looked to deep at the traffic.mis yet but that looks identical throught all project folders too so you should be getting traffic all over
The only thing which may differ is harbour traffic by project
Unless there is something only spawning in the mission area am not seeing
EDIT
Just to satisfy my curiousity ran the mission ed from Dec 41 after loading the Arctic convoys project folder
As you can see by the red squares and green circled ( random groups ) there is traffic about