In our airline normally the relief FO occupies the seat either P1 or P2 vacates. So in this case the P1 seat. It's assumed that the FO who is most 'senior' will become acting PIC.
As for flying roles: Depending on who is PF and PNF those ''roles'' stay with the seat.
eg. the P1 FO can be ''pilot flying and acting PIC''.
Whether this is what you want is another discussion and best left out of this accident.