As for changing the past, I think the past hasn't changed but the course may be taking detours. There is only one time line and anytime there is a course correction (something that should happen doesn't, so something else has to happen to get things back on track), memories are rewritten/added/erased. Remember in 5x02 when Charlotte said, ".. It's the weirdest thing earlier. I was thinking about my mum and all of a sudden I couldn't remember her maiden name. Isn't that odd?"
Do you suppose that, during a time jump, the Losties do somethign that causes time to temporarily shift courses, causing Charlotte's grandmother to not marry her grandfather (thus changing Charlotte's mum's maiden name)... but then of course, time "course corrects" and all is well in the future? Just havin fun
