The options I'm presenting myself with are either
(a) arrive by 1:30pm, spend the afternoon, evening, departing the town around 10:30am the next morning;
(b) arrive by 11:30am, spend the day, and depart around 4:30 pm (same day)
or
(c) arrive by 5:30pm, spend the night, then most of the following day, departing from the town around 4:00pm,
Options A and B both give me about 5 hours to explore, with option A letting me sleep in town.
Option C lets me sleep in town and then have a little more of a day there (depending on what time I get started after breakfast but also steals a few hours from Salzburg because I'd need to switch things around - it also adds an hour of train travel overall to my particular itinerary - on the day coming into Hallstatt). The risk with option C is that arriving this late - will I be able to get a room for the night? Rick says its hard to book a room in advance if only staying one night but can "fill in" when showing up for just one night.