Loads the Difficulty, Offering, Survivor and Slasher data that was saved by the lobby into SlashCo.CurRound, and builds the list of expected players for the round.
Assigns the loaded Slashers via SlashCo.SelectSlasher and, if the Nightmare offering is active, turns every Survivor into a Slasher instead.
Falls back to SlashCo.SinglePlayerSetup in singleplayer, or calls SlashCo.EndRound if the saved data could not be found.