The rules would be to reset the in game timer and play from Chapter 17 to the end credits.

Guide for how to reset IGT: ()

1. Use Chapter Select to start any chapter.
2. Choose to start without saving.
3. Quit to Main Menu. (now you have all chapters unlocked but no save file in place.)
4. Use Encounter Select to start any encounter.
5. Quit to main menu ASAP. (Encounter Select resets the IGT and when you quit there is no save file, therefore the IGT stays at 0:00.)

Now you can start from any chapter or encounter with the IGT at 0:00.

This also means that categories like Scotland (Ch 8&9) or Libertalia (Ch 12-15) are possible. Really any stretch of the game can be run separately.

Here is a test run done in 1:12:24 IGT and 1:20:40 real time: