An air engineer would need a significantly longer PET course than a navigator from any background.
RTFM!
It sounds as though the FSTA team have heeded the lessons of others. Another country proposes using pilots as MSOs to assist the boom operator when h/she is busy. When the RAF runs out of navigators, perhaps they might also need to consider using pilots. MSOs would only be required for AAR sorties, so why bother with an additional trade at all?