Excerpt taken from BBC h2g2 http://www.bbc.co.uk/dna/h2g2/A398955
"What if, when we had invented time machines, you went back in time, and accidentally landed on your grandfather, who had not yet had children. You would simply pop out of existence because you could not have been born. Then, you could not have killed Grandpa, and so he survives and you are eventually born and go back in time and land on him again, so you instantly pop out of existence... you would be stuck in an endless loop. There are a few theories around how this could be resolved."
What if the nemesis is from the future, and Jacob from the past. The nemesis travels backwards in time as a result of an as yet unidentified need to destroy Jacob. What isn't known to the nemesis initially is that his and Jacobs time-lines are connected such that when the nemesis kills Jacob (as seen at the end of series 5) the result is that an infinite time loop is created according to the grandfather paradox. The loophole would try to find a way to end the infinite nature of the loop they are stuck in. Maybe both parties have been able to communicate that they are stuck in the loop and are looking for ways to end it. Like groundhog day. Altering the dynamics of the time-line to enable a new ending. Unsticking a stuck record.
The rules are that you cannot kill someone with whom you have a connected time-line link. Maybe Ben and Widmore are connected in this way such that Widmore can klll Ben (his timeline isnt dependent on Bens existence), but Ben cannot kill Widmore (his timeline and existence is dependent on Widmore - they are related?) because to do so would create a new infinite loop.
Also from the same excerpt:
"End of the Universe
This theory involves two types of temporal loops. One type is the loop mentioned in the last paragraph, the 'grandfather paradox'. For the rest of this paragraph, let's call it the 'infinite repeat' loop, because it results in two different possibilities, infinitely repeating after one another. Another type of loop exists. It is the 'infinite possibilities' loop. In this loop, the loop changes every single time that the loop repeats. Think of this: Imagine that you ask your best friend to go back in time to before you were born and kill your granddad. Also, you had enough forethought to tell him, while he's back there, to write a note to his future self to go back in time and kill the man who would be your granddad. Everything's okay, right? Maybe not. When your friend is given the instruction to go and kill your granddad from you, he might do one thing. When he receives a note from his future self, he might do another. And if he does another thing during the second repeat, he must do a different thing the third. And the fourth. And the fifth. A change in one iteration of the loop would result in a change in the note, which would result in a change in the next iteration. Eventually, he'll do something that ends up breaking down the loop (ie, forgetting to write himself a note). This will result in a infinite repeat loop starting. And as was already mentioned, infinite repeat loops may cause the universe to end.
Fate
This theory allows for time travel to occur without any fear of paradox. But the price is not cheap. Under this theory, free will is an illusion. Nothing that you do is truly your own choice; everything was preordained at the beginning of the universe. If you accept this, then the 'grandfather paradox' vanishes. No matter how hard you tried, you could not go back in time and kill your grandfather. You would slip, or the gun would jam, or something else would pop up, but you could not kill him.
1. This theory allows for some interesting events to happen, however. Let's say that, while reading a history textbook, you noticed that a person who looked exactly like you saved George Washington during the Battle of Trenton. To meet this person, you go back in time. However, much to your dismay, you can't find them anywhere. But, since you're already in the past, you decide to see George. As you approach, you see that he is fighting the Battle of Trenton, and he is losing. You save him. When you return to the future, you ponder what has happened. Apparently, the history book was talking about you. Assuming that free will is an illusion, this could very easily happen.
2. Here's another example. One evening, you find a floppy disk on your desk which you're sure you didn't put there. You look at it, and sure enough, it's the blueprints to a time machine. So you build the time machine, and send the blueprints to your past self. Now, where did the blueprints come from? There is no answer to this question. The blueprints simply 'are'."
Option 1 is similar to the photo seen in 2007 for the DI with the losties in it.
Option 2 is as seen with Locke and nemesis - Locke receives orders from Richard and gives him his compass before time travelling. Richard says he will need to die. However this is as a result of the future time-line involving the nemesis, who already knows Locke will die, and he will take his form. The compass Richard has is given to the nemesis and the request for it further falsely proves to Richard that the nemesis is Locke
The infinite repeat loop seems to represent the outcome Eloise Hawking was talking about. Maybe the constant need to course correct ensures that infinite loop and end of universe scenario does occur. Something has changed in this iteration. Eloise apologises to Penny for Desmonds attack by Ben and blames her son Daniel. Daniel must have effected a change that has now altered the loop. The nuke may have prematurely neutralised the electromagnetic anomaly some 27 years before Desmond does. Those close to the neutralisation will experience the time travel phenomenon experienced by Desmond. They will appear in different time and spatial points and will need to establish a connection as Desmond had to. Maybe the reason for the white Lost screen at the end of the show.
As for jamming guns - Michael was shown that this may be whats occurring with the Lost world. He tried to shoot himself and wasnt able to. Other situations like this may have occured and explained away as the island being a living force, rather than the protection of time laws