hi
WCR is design mainly for raid (I dont do pvp myself), so all combat blocks detection/opponents are defined through raid flags.
So external raid players are hidden by default.
If you want to see pvp player (who are external raid player), you can add then manually.
-> Bring the context menu over the unit list (the right one)
-> submenu -> options uncheck Hide external player
-> all players are now shown in the list, and you can check them manually to add the one you need to see on the timeline
I am not sure the replay will work, however, I never tried ;)