Great story! I really like the way you chose to have Harry travel back in time, as opposed to the normal "Old Harry's mind in Young Harry's body" thing. That may happen with the soul merger, but he can set stuff up beforehand. It seems your Harry learned something from all those chess games with Ron.
I agree with twilliams1797. Harry's saving everyone this time around, he needs to help Luna's mom. He knows what it's like to grow up without a parent and is already working on making sure he and Neville don't have to go through that. He'd do whatever it took for Luna, as well. As for how Luna would act, probably almost as spacey (she is a Seer), but without the weirdness (Nargles, butterbeer cap necklaces, etc.) induced by too much exposure to the Quibbler without a mother's moderating influence.
Please continue this story. It is one of the best of it's type.