I think you mean FO or Capt. As soon as someone is incapacitated, the PM is also the PF, or vice versa.
Doesn't have to be flight deck, it could be one of the flight crew, meaning they need to divert and either source replacement crew, which at RNS is unlikely, or they unload pax and continue with a reduced load. Might not explain the 'rapid 180' but just throwing that out there