but would this be normal procedure for a FA to fill the vacant seat whilst FD crew was absent from the FD?
it is not uncommon for an airline SOPs to have the requirement for there to be two people on the flight deck at all times, so that if one of the flight crew needs to go elsewhere, one of the cabin crew will go stand in the cockpit. Whether that involves sitting in the hot seat is another matter, I'd say jump seat is more likely because it minimises the chance of bumping something important by accident. You have to weigh the chance of the FA needing to wrestle with the controls in the event that the PIC collapses compared to the need for the other pilot to get back into the chair real quick