Flash forward: Jack and Kate could have been only a few that left the island, Jack may want to go back for Juliet maybe. I think that Naomi was part of the crew that wanted to kill everyone on the island as Ben claimed, but they did not want to kill the people that crashed. Therefore rescued Jack and Kate, and the other survivers and killed or done something to the others.

Being one of the others, Juliet may be remaining on the island which will influence Jack to go back to there, and Kate does not want to go back because she pretty much hated all of the others. The funeral i believe it may have been michael or sawyer
Michael left the island out of selfishness we believe and may have killed him self in guilt
Sawyer had accomplished everything he wanted to...killing lockes father, after leading a miserable life he may have decided to kill himself.

Ben didn't say they'd be killed if they contacted Naomi's boat, he said they would die. I think this means they get the disease that Rousseau spoke of when the leave the island.

Juliette is my least favorite character. I'd like to see her thrown into a volcano on the island.