How is the accident, jughead, and killing jacob connected?
Daniel told Kate and Jack that they could be variables to change what happened so they are the only ones who could possibly change anything. Kate rejects this news and accuses Jack of acting like crazy-ol' John Locke because he accepts the news. So Jack must do something that will change what happened. I don't think he'll set off the bomb because the bomb never went off according to what Daniel told Richard a long time ago, but I do think he will attempt to stop the Accident back at the Swan station and be a victim of it.
In doing so, he becomes a time-warping entity like Desmond was after he triggered the safety lock of the Hatch (Flashes Before Your Eyes). This time-warping entity is known as Jacob. So if jack was Jacob, he would give orders that would ultimately make whatever happened, happen- Like Making lists, commanding Ben, and telling Christian/ Claire (his relatives) what to do.
Then in 2007, John Locke decides to kill Jacob/Jack who I have called "The Variable". In removing the variable, there are only constants. With only constants left in the equation, everything will return back to normal, thus the DI Losties will reunite with Sun, Ben, and Locke.