Lost Season 2 Finale?

The man that was freed by Michael said that Michael wouldnt be able to find the island after he left. Is it just unlikely that he'd find it again or will he literally never find it again? If so why not? What was the point of the bright light in the sky from using the key, and what would've happened if he had not used the key. Why are "The others" playing dress up? Do Walt or Michael ever return to the island. Are Mr Locke, Echo, and Desmond dead? Why is the camp deserted when Sayid scouts it? And finally, WHAT THE FRICK IS WITH THE CHESS PLAYERS, I dont understand that at all or why their computer acted up. Starting on Season three right now, any answers to any questions that I missed would be very much appreciated, though if an answer is coming up in the next few episodes, you can just forget about that question. Thanks a bunch!