But then we are going into a more "the island is magical" type explanation when it all can be explained logically (except for the connections between people, that TPTB said we don't need to explain).
Yes it is Nikki and Paolo who die but they aren't really dead:
(SPOILER)They get bitten by spiders which paralyze them and thus the castaways bury them, thinking that they are dead.
Also its obvious why Sun is talking to Sawyer like that. She finds out about her kidnapping in Season 2 by Charlie. If she tells Jin, Jin will kill sawyer for doing that to his wife. So instead she punches him.