Main confusion: Classic Reward one-way MEL-LHR in First is 192,000 points plus $618.72; one-way LHR-MEL is First is 192,000 points plus £552.36 (about $1,100 or nearly double the fees!). What explains the huge discrepancy? BTW, MEL-LHR return has fees of $1,564 which doesn't help.
Second...