Lost: Dead or not-Dead?

Now that it's out in the open, I figured I'd take a stab at a brief discussion about Lost and the all-consuming question of what happened to Oceanic 815 and where exactly are the fortunate survivors. Now that Locke's father has let the cat out of the bag, seems like a fair question.

Dead
--How else does John Locke get up and walk after 4 years in a wheelchair after getting thrown out an 8-story window?
--How else does the plane of Eko's brother find it's way to a tropical island after filing a flight plan that originated in Nigeria?
--How does Jack catch glimpses of his dead father on a tropical island, the same way Locke confronts his supposedly dead Dad?
--Why couldn't Desmond sail away from the island, to escape completely this strange place?
--We watched Mikhail die just a few weeks ago. How is it he was alive enough to administer first-aid to Desmond's would-be parachuting rescuer?

Not-Dead
--It doesn't adequately explain the Others. Why are they there? What purpose do they serve?
--If Desmond couldn't get away, how did Michael and Walt manage it?