Why? So far all time-jumping groups do so together. So if Jack, Kate and Hurley are ended up in the past, so should Sun and all the rest.
And she/they might be. But we don't know exactly how the time slips work or don't work so "for all we know," the people on the plane aren't bound by the same rules as the people on the island.
Jin could be in the present. He finds the van,gets it running,puts on some old Dharma clothes for something clean to wear, and drives around the Island for who knows how long.
Possible. But both the van and the uniform looked new.
