Uhura WAS fifth in command, at least before Chekov's promotion to Lt. Commander. It's painfully obvious that DeSalle was only commanding the bridge for the first and only time in CATSPAW as to prevent Uhura from having the seat, and preventing Dr. McCoy's ancestors from having an embolism while watching the TV. And as far as I remember DeSalle's rank was never mentioned in CATSPAW (his original appearance). Even if he was on the bridge, wasn't he merely navigator/ensign status?