Directly to the point...
Juliet failed to explode the bomb. The white flash at the end of the episode was the time travelling. Jack, Jin, Sawyer, Kate, Juliet, Miels (I do not know how exactly to spell it), Hurley and probably Bernard with Rose travelled back to their time. Cause for the time traveling is The Incident. The Incident is, as was suppose to be, due to drilling to the electromagnetic field. Miels'es theory that atomic bomb was the cause of The Incident is incorrect. Their attempt to intervene to the course of time had failed as bomb repeteadly refused to explode. (What happened happened)
When Jacob said that "They are coming", I believe he meant our Losties from 1977.
Regarding Mr.X i would like to say that I always thought that he uses the physical body of the dead people. But the latest episode showed that physical body of Locke is intact and is with the people of Jakob and Jakob himself requested the body to be brought to him.
I have a feeling that Jakob has outmaneuvered Mr. X.(again have a feeling that not the first time) I think he knew that Locke's incarnation is coming to kill him and that is why he requested Locke's body. His intention is to possess the physical body of him. So probably in the next season we will have two Locks, one being good (Jakob) and one being bad (Mr. X).
Under those two Locks will be created two camps which will fight with each other for the control over the island: Mr. X, Ben, Claire, Jack, Kate, Black Smoke (vs) Jacob, the Others, Hurley, Meils, Sawyer, Juliet, Jin, Sun.
Probably in the last episode of the 6th season it will be revelaed that Jacob and Mr. X were empowered safeguarding the island. But in the course of their cooperation some discripancies and misunderstandings arose between them (probably about the nature of the human beings and as always power issue) and epic struggle to conrol the island solely had started. As they could not touch each other (Rules imposed by people (or may be deities) who empowered them), they started to use people, who were brought to the island, to be their warriors.
I have a feeling that in the end of the show the powers which were given to Jacob and Mr. X will be taken away from them (probably their Masters will fed up with the constant struglle on the island) and will be given to Bernard and Rose.
Regarding the Losties, probably majority of them will die.
PS. Sorry for my enlish as it is not my native language.