One more day. One more day. One more day. I can make it. I came up with this theory a couple months ago and figured I'd better post it before "The Empty Hearse" comes out. (Squeeeeeee!) If you make it to the end and understand it, I congratulate you.
"Well then, about that dreadful chasm. I had no serious difficulty in getting out of it, for the very simple reason that I was never in it." says Sherlock Holmes in "The Adventure of the Empty House" when Watson asks how he managed to escape the abyss. For the purpose of this theory, I shall hold to that canon. In addition, there are innumerable things that may go wrong when jumping off a multi-story building, no matter how well-planned the event is. I do not think Sherlock would take such a risk. True, in "A Study in Pink" Watson comments that Sherlock would risks his life to prove he's clever, but we can see throughout the next five episodes how much that changes. Even by "The Great Game" this is evident.
Who, or what fell, then? Most theories offer Moriarty as a possibility, or a body from the morgue, using Molly's connections. I prefer the latter, for reasons I will expound on presently.
We must assume for this theory that Sherlock had assistance. As in the novels, I shall assume that he enlisted Mycroft as a confidant. Certainly scenes in "The Reichenbach Fall" indicate that he requested help from Molly. What if, though, in that missing scene, he was just giving Molly false information to throw everyone off? He needs those close to him to believe in his death. However, that possibility does not work very well with my theory, so I'll ignore it. Real scientific I'm getting here.
Many have suggested the possibility of assistance from the homeless network. There, I simply must disagree. We have seen in previous episodes that while Sherlock is willing to use the network for investigations, he doesn't exactly trust them. Also, in "The Final Problem", Holmes describes Moriarty as a spider, sitting motionless at the center of his web, receiving signals from his agents, who are "numerous and splendidly organized". If Sherlock knows this, then he would certainly be aware of the danger of a double-crossing agent in his homeless network. Again, too much of a risk.
Here we come to possibly the only unique part of my theory. I'm not even going to try to say how much Sherlock knew, or how much he was planning. But let us assume that he knew Moriarty was going to confront him, and probably kill him. Let us also assume that he was aware that the keycode was a fake. So here we have Sherlock, sitting and planning what to do. He needs John gone, which is easily accomplished, and he wants to have as much of an advantage as possible, so he chooses their meeting place. I don't think Sherlock knew exactly how Moriarty planned to kill him, so I assume that he planned for many different scenarios. Maybe he was wearing a bullet-proof vest. I don't know. . .
Sherlock is thinking of ways to cheat death. Certainly something that comes up is Irene Adler and her faked death. (The first one, not the second one that he had a hand in). So he needs a body and someone who can fake documents. Enter the ever helpful Molly Hooper. They rig up a body to look like Sherlock, complete with coat, etc.
Of course, thinking of Irene Adler would make Sherlock think of other events in "A Scandal in Belgravia". Really, that entire episode is about cheating death. First with Sherlock and Watson escaping from Moriarty thanks to chance, then from the agents with vatican cameos. Irene Adler fakes her death, then comes the whole Coventry brouhaha, which is cheating the terrorists of their victims. Finally comes Irene's second 'death'.
Hmmm. . . Coventry. The flight of the dead. Let's see where this leads us. Mycroft's Flight of the Dead can be summed up easily: Instead of revealing that they have cracked the terrorist's code, the government sends a plane of dead bodies to be blown up in midair. Hundreds of casualties, but no one dies.
Let's replace a couple of words in that summary and see what happens: Instead of revealing that Sherlock has cracked Moriarty's keycode, he sends a dead body to be killed in a fall. A dead body, but no one dies.
Are you following so far? I'm incredibly bad at explaining things.
You may be wondering, how on earth does Sherlock shove a body off without it looking odd?
In Alan Bradley's novel "The Sweetness at the Bottom of the Pie" there is a similar scenario. (If you haven't read the book, there are spoilers ahead. Go read the book. It's fantastic). A subplot includes the alleged suicide of a man. The man, by all eyewitness accounts, appears to have stood at the top of a tall tower, shouted "Vale!" then leaped to his death. However, it eventually discovered that the 'suicide' was actually a murder. The murderers had killed the man with a blow to the head, then one of the murderers dressed in identical clothes to the victim, then stood and shouted the farewell. He then ducked behind a parapet while his accomplices shoved the body over. To the sun-blinded observers, the man killed himself. Look here at the screencap from one of the final scenes.
Confusing? Probably. Possible? Definitely. Accurate? We'll see.