I was waiting to post this theory to see if last night's episode had any new information in it. But alas, "What Kate Does" seemed to be mainly just a filler episode, thus my theory still remains the same. I haven't posted in the official "Theory" section before, though I have been reading the theories on this site almost since it began. Of course, if anyone else has already posted this, let me know!
This theory tries to explain a few things that had previously been, to be quite frank, big questions in my head. Mainly: why is Christian out and alive on the Island, why would Desmond not pushing the button make the plane break IN HALF (I understand it crashing, but breaking in half??), and finally why in the season premiere "LA X" was Jack's dad not present on the plane as well as Locke's knives. I think it's important to note that in the original timeline Christian's coffin and Locke's knives both ended up on the Island, whereas in the ALT they both appear to be missing. They almost seem "linked" in some way, like they go with each other.
We know that on the Island Christian is alive again. Now I've heard a lot of theories that Christian is either a manifestation of or being controlled by either Jacob or the monster/MIB. But I don't think either of these make sense, saying Christian's actual body is missing (not a copy), and especially in light of the newest episode in which the Others claimed that Sayid and Claire were both "claimed" individuals. This is the basis of my theory: Christian was never dead. How? They could have faked it, or maybe the Island's powers resurrected him. That's the part I don't think we have enough information to know for sure yet. So what does that have to do with the other points I brought up?
Well why would Christian's coffin not be on the plane? It could be explained by a mix of Christian being alive and Locke's missing knives. Think about it, if Christian were alive he'd obviously want to get out of the coffin and leave. And how could Christian break out of the coffin? Using Locke's missing knives! I mean, unless he's just super strong and punched his way out (sarcasm there). This would also explain why he's on the Island in the original timeline. If he were alive, he'd want to get out, but in the original timeline he wasn't able to get to Locke's knives and thus couldn't get out of the coffin until they crashed on the Island. Maybe the crash loosened the hinges and allowed his physical body to leave. Either that or he was able to get out while still in the cargo (where Locke's knives were as well!) and that's why he's not with his coffin on the Island originally.
So with all this knowledge we can tackle the last question I raised: why did the plane break in TWO? Well okay, this last point may be a bit of a stretch, but it's something that's always bugged me and I'm trying to explain with this. Any better ideas, let me know. But let's say Christian actually did get out with Locke's knives in the other timeline while still in the plane's cargo. So he already wants to get out of there, add to that the fact that he's in the cargo, so he would REALLY want to get out. Well maybe around the time Desmond didn't push the button Christian did something. I don't know exactly, maybe loosened the hinges or maybe cut a hole in the hull and made the plane at least more likely to break into two parts.
I'll admit, the last point needs some work... but yeah. So thanks for reading! I've been trying to work on this theory for a while, and would love some truthful feedback! CANNOT WAIT to see how this amazing show we call LOST ends! My life isn't gonna have as much purpose, I know that! :-)