The game was too dark for me until I went into the setup screen and changed the gamma correction setting to medium; it's now perfect. It still has the gloomy atmosphere but it's easy to see everything.
I would imagine that a new/revised/additional walkthrough will need to be written for this game, not only for the new chapter but also a new walkthrough for the base game, as apparently new puzzles have been added (among other things).
It's my understanding that the new chapter "The Last Visit" takes place six years after the events of the main game; so, yes, that section of the game should be played last. Have fun!!