May not actually be the biological father of Walt, as hinted by Ms. Klugh, who took a blood sample from Michael when he arrived at their camp.
Michael and Sun had a relationship on the Island. This has been hinted in many episodes and also a mobisode. The Others knew about the affair, indicated in "Meet Kevin Johnson" when Tom realizes that Michael was having cold feet about blowing up the boat. Tom starts to mention names of survivors who would die if he didn't blow up the boat. He mentions Sun first to give Michael an emotional response
Hurley only said "Like your dad, yeah" to Walt because he doesn't want to tell Walt that Michael is dead.
In addition Walt tells Hurley that he doesn't understand why they have to lie and is told it is to protect those that didn't come back. If Walt knew that that did NOT include his father he might feel less inclined to comply, but since he thinks he is also protecting his father who he seems to want to see again he will keep up the cover story etc.
Inability to die
The Island wouldn't allow Michael to die or kill himself prior to joining the freighter crew wasn't due so much to the Island itself not allowing it, but because it was already known to happen that way. Now that it's known that several people from the "modern" timeline lived on the island during the 70s after time traveling there, it was already known that the events that caused the people from the modern timeline to ultimately end up living in the 70s was partially a result of events that happened on the freighter with Michael's involvement.
The die-hard fatalist would say that the Island was responsible for the structure of the time-line itself.
Maybe "the island preventing anyone from doing particular actions" thing is just a metaphor for knowledge of future events. Someone, like Jacob, might be from the future (let's assume that he is Aaron) and can pretend to anyone that the island won't let you do what you want, not because the island as such power, but because whatever happened in his future time, has to happen.
If Jin is alive after the freighter exploding, it is possible Michael is too.
Highly unlikely. Jin survived because the blast threw him into the water, but Michael was in the same room with the big pile of C4. No way to survive that, unless he's still immortal, which seems unlikely after Christian's comment and his subsequent non-appearance.
His appearance among the whispers confirms his death on the freighter.
Perhaps Jacob's nemesis has the ability to make people immune to death. Hence his appearance as Christian Shephard at Michael's last moment, announcing the end of this immunity.
Christian Shephard appeared to Michael just before a crucial moment, similar to the one with John Locke before he turned the Frozen Wheel and jumped into the future. Christian's line "You can go now" is somewhat ambigious. Go WHERE? To the future? ("You can die now" would have been more appropriate and hint Michael the possibility that he had redeemed himself).
Michael's inability to die or be killed is reminiscent of that of a time traveler arriving from the future. If the freighter explosion sent him to the future and he would 'later' travel back to a point in time before the freighter explosion, he could not kill himself or be killed as this would otherwise constitute a temporal paradox (The huge problem, of course, is that it would have to be the Michael from the Future trying to kill himself and couldn't explain what happened to island Michael).
Jacob would not let Michael die until he served his purpose. We learn in "Dr. Linus" (season 6) that Richard Alpert cannot kill himself, and so he asks Jack to light the dynamite in the Black Rock for him. Jack lights the dynamite but refuses to leave the ship, and sure enough, the fuse burns out -- just as the gun misfired when Michael tried to shoot himself in the head, and just as Kearney's gun misfires when he tries to shoot Michael on the boat -- because Jack (and/or Richard) is still under Jacob's protection. Michael's "tasks" appear to have been limited to providing Linus with the names of the people on the boat and saving the lives of Jin, Sun, Lapedis and/or Desmond - none of which would provide any apparent benefit to Nemesis/MiB. The fact that Christian was the one to tell Michael his service was complete suggests Christian actually is on Jacob's side -- if Nemesis/MiB were able to materialize on a ship off the coast of the island, then his complaints of being trapped there and wanting to leave would make no sense.
richard couldnt kill him self because people who were blessed by jacob were not able to kill them selves,the dynamite didnt work because jack him self was unable to kill him self,its one of the rules
Inconclusive: Since we do not know how long MiB can retain his hovering smoke monster form, he may be able to cross a certain distance over the water but not across an ocean. Likewise Michael died on the Kahana but his ghost / spirit was able to travel back to the island.