"…a nut on a component that moves the left elevator had fallen off…" really doesn't provide much info at this point. It could've been any number of components.
Just as a basic anatomy lesson on the MD80 series, the elevators themselves are control-tab operated and not connected to the control columns…or each other. At the gate, it's normal (if the wind is from the tail) to see one elevator in the UP position and the other in the DOWN position.
The only hyd source to the elevators is the elevator augmenter system which isn't a normal operating mode but rather a deep stall recovery effort which uses hyd pressure to push both elevators full down if the control tabs are moved beyond a certain amount in the ANU direction. The ELEVATOR PWR ON light comes on to indicate that hyd pressure has been applied to the elevators…not where the elevators went. This light is observed during the normal control check during taxi by pushing the control column full ANU. This will normally move both elevators ANU which cannot be seen by the crew during taxi. The system has an accumulator so it can move the elevators at the gate if you're inclined to play with it to see it work.
There is no flight control position indication in the cockpit (other than stab trim position) like some airplanes have.
As for the Allegiant situation, the crew wouldn't have been able to see anything abnormal on a walk-around. Before castigating the crew, it might be prudent to wait for a lot more information about this.
I have no idea what would've happened if they'd taken the airplane into the air with one elevator jammed full ANU.