THIS IS MY THEORY. (Such as it is, I admit it needs work.)
Jim Moriarty employed a body-double (plastic surgery enhanced?) of Sherlock's to kidnap the ambassador's kids, which is why the little girl was so terrified when she saw him. Sherlock deduced that this must be the case, and at some point off-screen managed to track him down (between when he separates from John and goes to see Molly?). Jim, being Jim, at some point would have disposed of the double, to tie up loose ends; Sherlock would find the man's body. Knowing he's unlikely to be able to avoid dying (or "dying") due to Jim machinations, Sherlock sets up his plan with Molly Hooper. He puts on a bit of a show for Jim's men who are watching him, waiting for him to die, waits for John to come back so he can give him his "note", makes him stand in a very specific spot so that he has very specific sightlines, and jumps off the roof. He lands in the open-topped garbage truck that he's paid to be parked right there, while the body-double is rolled out of the truck and onto the sidewalk. As John runs forward, we can clearly see that the garbage truck had obscured his view of the patch of sidewalk where he believes Sherlock landed. The biker is working for Sherlock, as is the truck driver– and possibly some or all of the other people as well. Biker knocks John over, taking John's eyes off the scene for a few seconds and leaving him dazed from a minor head injury. We then see the truck pulling away, presumably with the real Sherlock inside it. The crowd, intentionally or not, runs interference, not leaving John much time with the body, and of course John's still about wobbly in the brain pan. Molly Hooper, coroner extraordinaire, does the necessary faking and lying (with Sherlock's help) so that everyone is convinced he is dead.
...Okay so it's nowhere near perfect, the part that I still can't quite make work for me is the body-double timeline. I would believe that Sherlock had time to find him, but if he's been dead too long he wouldn't be warm anymore and he would've gone into rigor mortis, and even if Sherlock paid everyone else off John touched him. But I can't see Sherlock being responsible for his death, which I sort of think he'd have to be if he was killed just minutes prior. Maybe there's a way to fake recent death? Maybe it's been 48-60 hours since he died and rigor mortis has worn off, and they warmed his body up a little aslghalkshgsjf idek. I REALLY DO THINK BODY-DOUBLE IS THE ANSWER, IT EXPLAINS THE LITTLE GIRL AND WHY ELSE WOULD THE TRUCK DRIVE AWAY. I suppose it's possible Sherlock jumped into the truck, jumped out, and played dead BUT BUT BUT PULSE. Body-double theory confuses me slightly but still seems neater somehow, fits the facts better.
...I LOOK FORWARD TO BEING COMPLETELY WRONG AS USUAL. :D :D
ETA: NO WAIT. WAIT. I'VE CHANGED MY MIND. I didn't realize it was possible to stop your wrist pulse by squeezing a ball in your armpit. I THINK THIS THOROUGHLY PUNKS MY BODY-DOUBLE THEORY. Because because because there's that ball he keeps playing with at the morgue. The garbage truck is still totally part of the plan, though.