It autosaves when a game finishes (mate, resign, adjudicate). But not when you abort a game by pressing 'New Game'. Btw, an alternative workaround might be to install POS with the extra WInBoard option -firstFeatures "myname=POS". This should overrule the myname feature that POS is sending itself, and allows you to select an aceptable name.
You don't have the --brain-mode within the double quotes that surround the pos.exe, which makes it a WinBoard option, and not a POS option.
When you just rin WinBoard with Additional option -recentEngineList "" to clear it, you can get the other two engines back in it quickly enough by selecting them from the listbox in the Load Engine dialog.
WinBoard has an option -initialMode MachineBlack that could be helpful. It seems you have multiple games in the PGN file; it might be that adding an -lgi N option automatically loads the Nth game from it. (I had no multi-game PGN files at hand to quickly try it.)