Good Morning DW,
...handedness is not an issue, and as it seems relief pilots can sit in either seat it's not like F/Os get no practice with it on that side.
All the hand flying practice will have been done from the co-pilot's natural position (RHS). During cruise relief, when seated in the LHS - all inputs will have done through the auto-pilot.
Occasionally, in the simulator the co-pilot will get an opportunity to hand fly from the LHS - and from what I've observed, some find it much more difficult. Try playing tennis with your non natural hand and let me know how you get on.
The side stick is a perfect device for controlling the aircraft in Normal Law (nudge to attitude and let go), but is not ideal for controlling Roll in Alternate Law (it's too sensitive and needs constant pressure until the aircraft is perfectly trimmed).
The conventional control column is able to accept both hands and is more natural than the side stick from either seat.
Edit. If there was so little difference between the handedness of the side sticks, seat positions etc., why do you think it is mandatory for Training Captains (who can occupy either seat) to additionally perform EFTO & GA procedures from the co-pilot seat in the simulator? If there was so little difference - we wouldn't bother.