1. He means something about her mouth implies that she is strong - not physically, but instead that she is tough or maybe ethically upstanding.
2. Yes, strong is a compliment, with same meaning as number 1.
3. "The gathering night" is a fancy way of saying "it is getting dark" or "the sun is setting." I dont think "gathering" is used that way in any other context, though it probably was 500 years ago.