I think it was pretty obvious Hurley was full of shit when he 'confirmed' it. Watch the scene again, he clearly looks uncomfortable when Walt asks him that question. In terms of the 'you can go' line, that one seemed pretty obvious to me: Tom had told Michael that the island would not let him die until it was finished with him, a fact that Ben later reiterated. I think it's pretty clear that 'you can go now' is Christian telling Michael he can finally die, as he wanted to all along. Christian, I'm convinced, is an apparition of some sort, not a legitimately tangible being.
In terms of Jin's having to die at some point, I think you missed the whole point. As far as Sun is concerned, Jin died on the freighter. That's what she saw and she's yet to be presented with evidence otherwise OFF the island. That doesn't confirm his demise in any way, shape or form, however. I'm banking that he either made it back to the island before it was moved (Clearly there's a perimeter to the island outside it's actual above ground portion, or it'd somehow leave stuff like the Looking Glass station behind), which Sun would have no way of knowing about, or he is in hiding off island so as not to contradict the Ocean Six's story. If he survived, but didn't meet up with them before the story got out, he likely couldn't bring himself to approach Sun, knowing that if they remained together it would out their story, dooming everyone, or ruin Sun's and Ji Yeon's lives as they'd need to go underground with him.