PJ2, re "RP".
AFAIK, AF doesn’t use "Relief pilots" per se. Long haul F/Os are always ATPL licensed, fully type and line qualified. The captain selects the F/O in charge during his rest period, usually the most senior (they often have equivalent seniority). This F/O is to take the RH seat.
Mr Optimistic, re "The a/c didn't deviate like others"
No two a/c deviated identically that night. Storm cells have a tendency to move. There may be a radar issue though, just as anything else.