Yeh Jin could've just jumped off the boat as you didn't see him on the deck when it blew. Hurley playing chess with Mr. Eko was quite random. I think Sun must've some how found out Jin was still alive, I don't think she'd team up with Whitmore to save Juliet and co.
This whole moving of the island thing, i'm wondering how many times it's happened before. The Dharma team constantly referred to an 'incident' in the past which fucked them over basically. It'd make sense the others move the island after they killed off Dharma. Certainly explains the Black Rock, it must have been in the center of the island when it rematerialised.