I hadn't considered the possibility of a Vic diverting via Palace to make the staff stop at Norwood Jn instead, I suppose when that happens there is a third pair of units out and presumably at least one extra driver and guard. As you say, the diagrams are tight with little turnround time at Three Bridges and Vic.
Are the side entrances to all the intermediate shacks actually locked after the last train then? I had always thought that the booking offices were closed and the side entrances left open while the station was unstaffed.